Update solang and canvas-node in contracts CI image

[TriplEight]

Can't be merged until paritytech/canvas#21 is resolved, this will break the test job in cargo-contracts CI. Other than that, fmt and build are successful.

Temporary workaround: use-ink/cargo-contract#114

• solang update to v0.1.2, newer versions did not work
• canvas-node binary included as an artifact from the latest master
• clang-8 set as a linker
• python3 set as default

Closes paritytech/ci_cd#1
Closes paritytech/ci_cd#18

[TriplEight]

@ascjones @cmichi @Robbepop how to proceed with it? Is there any progress with paritytech/canvas#21

[ascjones]

I thought that paritytech/canvas#21 is solved by this PR, because the binary no longer is compiled by this image, instead it is now downloaded from the result of the canvas-node CI which currently runs successfully.

[TriplEight]

@ascjones sort-ta. There's no problem to build cargo-contract within this image, I specifically tested this.
But the test job of cargo-contract CI would fail on the same error: could not compile sp-arithmetic.

[ascjones]

Summary:

• cargo-contract depends on the latest release of substrate 2.0.0 on crates.io
• substrate 2.0.0 fails to build with nightly > 2020-10-01
• This PR produces an image which contains a newer (incompatible) version of nightly

Therefore this PR should not be merged until a new version of substrate is available on crates.io which can compile with the latest nightly, and cargo-contract has been upgraded to depend on that version.