[Fix] 'nvm exec' no longer prints error about '-q' being an invalid option

The `NVM_CD_FLAGS` variable is assigned a value if nvm is being run in
zsh, so that `cd`'ing around won't print any error messages. This flag,
however, is not compatible with bash's `cd` built-in.

Because this variable was exported, it meant that a user running zsh
would be unable to run `nvm exec ...` commands without encountering
errors.

This issue manifests itself in the following manner:

```console
$ nvm exec 16.14.0 npm --version
Running node v16.14.0 (npm v8.3.1)
/Users/<ME>/.nvm/nvm.sh: line 28: cd: -q: invalid option
cd: usage: cd [-L|[-P [-e]] [-@]] [dir]
both the tree and the node path are required
N/A: version "v16.14.0 -> N/A" is not yet installed.

You need to run "nvm install v16.14.0" to install it before using it.
```

> in this case, node 16.14.0 _is_ installed.

By not exporting the variable, it will not be visible inside the
`nvm-exec` script, and the `NVM_CD_FLAGS` will be assigned fresh on each
run when the `nvm.sh` file is sourced, and commands like this will be
successful.

Author: spikegrobstein

Diff for: nvm.sh

@@ -332,7 +332,7 @@ nvm_install_latest_npm() {
 # Make zsh glob matching behave same as bash
 # This fixes the "zsh: no matches found" errors
 if [ -z "${NVM_CD_FLAGS-}" ]; then
-  export NVM_CD_FLAGS=''
+  NVM_CD_FLAGS=''
 fi
 if nvm_is_zsh; then
   NVM_CD_FLAGS="-q"