feat: template-injection: filter static envs

[woodruffw]

This eliminates a class of false positives in the template-injection audit, where ${{ env.foo }} was flagged as a potential injection source despite foo being defined as a literal (non-expression) string in one of the workflow's env: blocks.

More generally, there's still a group of false positives this doesn't address. For example:

run: echo "${{ env.foo }}"
env: 
  foo: ${{ true }} # or anything else safe

This still flags ${{ env.foo }} as an injection source, despite foo's value coming from a "safe" template expansion.

Closes #313.