Feature request: Add a pre-gap input to allow for gap to be run within a different tool

[Joseph-Edwards]

When checking for memory leaks in test_file.g, I run valgrind --trace-children=yes --leak-check=full gap test_file.g. It'd be nice to be able to pass a command that should be prepended to the variable $GAP. In this example, this would be invoked with something like

...
  - name:
    uses:  gap-actions/run-pkg-tests@v2
    with:
      GAP_TESTFILE: "test_file.g"
      pre-gap: valgrind --trace-children=yes --leak-check=full