Wrong Gcov Executable in Test Workflow

[threeal]

This issue identifies this failing build on the latest master, which appears to be caused by incorrect usage of the Gcov executable when calling the Gcovr Action.

Since #87, the test workflow is no longer built using llvm, but the Gcovr Action is still utilizing the LLVM version of the Gcov executable. This mismatch has resulted in a failing build since the merging of #88.

This issue recommends reverting the Gcov executable back to using the default executable.