travis: Delay sccache downloads in docker

Let's have this layer be towards the end as we're emprically changing sccache
more than we're changing the rest of the image, so this'll allow us to reuse as
much of the cached image as possible.

Author: alexcrichton

src/ci/docker/dist-arm-linux/Dockerfile

@@ -27,10 +27,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   libssl-dev \
   pkg-config
 
-RUN curl -o /usr/local/bin/sccache \
-      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
-      chmod +x /usr/local/bin/sccache
-
 RUN curl -OL https://github.com/Yelp/dumb-init/releases/download/v1.2.0/dumb-init_1.2.0_amd64.deb && \
     dpkg -i dumb-init_*.deb && \
     rm dumb-init_*.deb
@@ -65,6 +61,10 @@ RUN ./build-toolchains.sh
 
 USER root
 
+RUN curl -o /usr/local/bin/sccache \
+      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
+      chmod +x /usr/local/bin/sccache
+
 ENV PATH=$PATH:/x-tools/arm-unknown-linux-gnueabi/bin
 ENV PATH=$PATH:/x-tools/arm-unknown-linux-gnueabihf/bin
 

src/ci/docker/dist-armv7-aarch64-linux/Dockerfile

@@ -27,10 +27,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   libssl-dev \
   pkg-config
 
-RUN curl -o /usr/local/bin/sccache \
-      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
-      chmod +x /usr/local/bin/sccache
-
 RUN curl -OL https://github.com/Yelp/dumb-init/releases/download/v1.2.0/dumb-init_1.2.0_amd64.deb && \
     dpkg -i dumb-init_*.deb && \
     rm dumb-init_*.deb
@@ -66,6 +62,10 @@ RUN ./build-toolchains.sh
 
 USER root
 
+RUN curl -o /usr/local/bin/sccache \
+      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
+      chmod +x /usr/local/bin/sccache
+
 ENV PATH=$PATH:/x-tools/aarch64-unknown-linux-gnueabi/bin
 ENV PATH=$PATH:/x-tools/armv7-unknown-linux-gnueabihf/bin
 

src/ci/docker/dist-powerpc-linux/Dockerfile

@@ -27,10 +27,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   libssl-dev \
   pkg-config
 
-RUN curl -o /usr/local/bin/sccache \
-      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
-      chmod +x /usr/local/bin/sccache
-
 RUN curl -OL https://github.com/Yelp/dumb-init/releases/download/v1.2.0/dumb-init_1.2.0_amd64.deb && \
     dpkg -i dumb-init_*.deb && \
     rm dumb-init_*.deb
@@ -66,6 +62,10 @@ RUN ./build-powerpc-toolchain.sh
 
 USER root
 
+RUN curl -o /usr/local/bin/sccache \
+      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
+      chmod +x /usr/local/bin/sccache
+
 ENV PATH=$PATH:/x-tools/powerpc-unknown-linux-gnu/bin
 
 ENV \

src/ci/docker/dist-powerpc64-linux/Dockerfile

@@ -25,11 +25,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   wget \
   xz-utils \
   libssl-dev \
-  pkg-config
-
-RUN curl -o /usr/local/bin/sccache \
-      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
-      chmod +x /usr/local/bin/sccache
+ pkg-config
 
 RUN curl -OL https://github.com/Yelp/dumb-init/releases/download/v1.2.0/dumb-init_1.2.0_amd64.deb && \
     dpkg -i dumb-init_*.deb && \
@@ -70,6 +66,10 @@ RUN apt-get install -y --no-install-recommends rpm2cpio cpio
 COPY build-powerpc64le-toolchain.sh /tmp/
 RUN ./build-powerpc64le-toolchain.sh
 
+RUN curl -o /usr/local/bin/sccache \
+      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
+      chmod +x /usr/local/bin/sccache
+
 ENV PATH=$PATH:/x-tools/powerpc64-unknown-linux-gnu/bin
 
 ENV \

src/ci/docker/dist-s390x-linux-netbsd/Dockerfile

@@ -27,10 +27,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
   libssl-dev \
   pkg-config
 
-RUN curl -o /usr/local/bin/sccache \
-      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
-      chmod +x /usr/local/bin/sccache
-
 RUN curl -OL https://github.com/Yelp/dumb-init/releases/download/v1.2.0/dumb-init_1.2.0_amd64.deb && \
     dpkg -i dumb-init_*.deb && \
     rm dumb-init_*.deb
@@ -69,6 +65,10 @@ RUN ./build-netbsd-toolchain.sh
 
 USER root
 
+RUN curl -o /usr/local/bin/sccache \
+      https://s3.amazonaws.com/rust-lang-ci/rust-ci-mirror/2017-03-16-sccache-x86_64-unknown-linux-musl && \
+      chmod +x /usr/local/bin/sccache
+
 ENV PATH=$PATH:/x-tools/s390x-ibm-linux-gnu/bin:/x-tools/x86_64-unknown-netbsd/bin
 
 ENV \