ci: use hash of master branch for Bazel cache key

[brandonroberts]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[x] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #

What is the new behavior?

Each PR pulls the bazel cache based on the latest SHA from the master branch on this repo. This should reduce the time for Bazel to build packages in CI that haven't changed, as it will reuse that cache. Each time a PR is merged, a new immutable cache is built based on that SHA.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information