[llvm] [Support][Jobserver][Tests] Simplify default executor init and make (PR #165264)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-sie-win` running on `sie-win-worker` while building `llvm` at step 7 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/46/builds/26346

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 7 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'LLVM :: DebugInfo/PDB/pdb-invalid-type.test' FAILED ********************
Exit Code: 3221225477

Command Output (stdout):
--
# RUN: at line 1
z:\b\llvm-clang-x86_64-sie-win\build\bin\llvm-pdbutil.exe yaml2pdb -pdb=Z:\b\llvm-clang-x86_64-sie-win\build\test\DebugInfo\PDB\Output\pdb-invalid-type.test.tmp1.pdb Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\DebugInfo\PDB/Inputs/one-symbol.yaml
# executed command: 'z:\b\llvm-clang-x86_64-sie-win\build\bin\llvm-pdbutil.exe' yaml2pdb '-pdb=Z:\b\llvm-clang-x86_64-sie-win\build\test\DebugInfo\PDB\Output\pdb-invalid-type.test.tmp1.pdb' 'Z:\b\llvm-clang-x86_64-sie-win\llvm-project\llvm\test\DebugInfo\PDB/Inputs/one-symbol.yaml'
# note: command had no output on stdout or stderr
# error: command failed with exit status: 0xc0000005

--

********************


```

</details>

https://github.com/llvm/llvm-project/pull/165264