[CI] GitHub Actions cache for `_deps` should be automatically invalidated when there's source changes to `CMakeLists.txt`, `ci.yml`, etc.

[saki7]

Currently the only "key" included in our cache setting is basically limited to the toolchain's version numbers:

iris/.github/workflows/ci.yml

Line 148 in ff27277

key: deps-${{ matrix.os.name }}-${{ matrix.os.version }}-${{ matrix.compiler.toolset }}-${{ steps.env-info.outputs.compiler-full-version }}-${{ matrix.cpp_version.name }}-${{ matrix.build_type.name }}

I experienced several times where invalid cache is persistent in some pull requests, resulting in a complete rebuild of Catch2 even that the CI "finds" the (invalid) cache.