launcher: Use actual executable path instead of `argv[0]`

[fmeum]

When Jazzer is executed from PATH, argv[0] will just be jazzer, which we can't find jazzer_standalone.jar relative to.

Also fix argv[0] in the case where no shell launcher is generated.

Fixes #641

[fmeum]

The code path is also used in all Bazel tests now as we pass the result of the new function to the C++ runfiles library. But I will try to add coverage for PATH invocation specifically.