[SYCL][E2E] Substitute paths to lit tool binaries

[frasercrmck]

This change makes LIT explicitly substitute uses of tool binaries in
tests with their full paths. This makes test RUN lines easier to
reproduce, as it negates the effect of lit's internal environment (e.g.,
PATH) on which tools are run.

The list of tools affected are: sycl-ls llvm-spirv, llvm-link, FileCheck
and not (when used after a pipe).

This also changes the behaviour of how LIT finds sycl-ls, llvm-spirv,
and llvm-link. Whereas before these tools were found only on the PATH,
now they will be preferably sourced first from the in-tree tools
directory.

[frasercrmck]

@aelovikov-intel might be of interest to you again.

[frasercrmck]

Hmm, I now see that pre-built tools should already be picked up in tests by this line:

llvm_config.with_environment('PATH', config.sycl_tools_dir, append_path=True)

but later on in the cfg we still have manual checks and warnings for llvm-spirv and llvm-link which don't take this into account, so we're unnecessarily disabling tests if these tools are present in the tools directory but aren't on the PATH.

[frasercrmck]

I'm not sure if it's normal to wait over 6 hours for an AMGPU runner but rebasing this PR looks to have squashed all of the failures we were previously seeing.

[aelovikov-intel]

@intel/llvm-gatekeepers , this PR is ready.