[llvm] [cross-project-tests] Use lit internal shell (PR #203138)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `llvm-clang-x86_64-darwin` running on `doug-worker-3` while building `cross-project-tests` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/23/builds/20331

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

```
Step 6 (test-build-unified-tree-check-all) failure: test (failure)
******************** TEST 'cross-project-tests :: debuginfo-tests/llgdb-tests/nested-struct.cpp' FAILED ********************
Exit Code: 127

Command Output (stdout):
--
# RUN: at line 3
/Users/buildbot/buildbot-root/x86_64-darwin/build/bin/clang --driver-mode=g++ --target=x86_64-apple-darwin23.6.0 -O0 -g /Volumes/RAMDisk/buildbot-root/x86_64-darwin/llvm-project/cross-project-tests/debuginfo-tests/llgdb-tests/nested-struct.cpp -c -o /Volumes/RAMDisk/buildbot-root/x86_64-darwin/build/projects/cross-project-tests/debuginfo-tests/llgdb-tests/Output/nested-struct.cpp.tmp.o
# executed command: /Users/buildbot/buildbot-root/x86_64-darwin/build/bin/clang --driver-mode=g++ --target=x86_64-apple-darwin23.6.0 -O0 -g /Volumes/RAMDisk/buildbot-root/x86_64-darwin/llvm-project/cross-project-tests/debuginfo-tests/llgdb-tests/nested-struct.cpp -c -o /Volumes/RAMDisk/buildbot-root/x86_64-darwin/build/projects/cross-project-tests/debuginfo-tests/llgdb-tests/Output/nested-struct.cpp.tmp.o
# note: command had no output on stdout or stderr
# RUN: at line 4
PYTHON_EXEC_PATH=/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/bin/python3.9 LLDB_PYTHON_PATH=/Users/buildbot/buildbot-root/x86_64-darwin/build/./lib/python3.9/site-packages /Users/buildbot/buildbot-root/x86_64-darwin/llvm-project/cross-project-tests/debuginfo-tests/llgdb-tests/test_debuginfo.pl /Volumes/RAMDisk/buildbot-root/x86_64-darwin/llvm-project/cross-project-tests/debuginfo-tests/llgdb-tests/nested-struct.cpp /Volumes/RAMDisk/buildbot-root/x86_64-darwin/build/projects/cross-project-tests/debuginfo-tests/llgdb-tests/Output/nested-struct.cpp.tmp.o
# executed command: PYTHON_EXEC_PATH=/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/bin/python3.9 LLDB_PYTHON_PATH=/Users/buildbot/buildbot-root/x86_64-darwin/build/./lib/python3.9/site-packages /Users/buildbot/buildbot-root/x86_64-darwin/llvm-project/cross-project-tests/debuginfo-tests/llgdb-tests/test_debuginfo.pl /Volumes/RAMDisk/buildbot-root/x86_64-darwin/llvm-project/cross-project-tests/debuginfo-tests/llgdb-tests/nested-struct.cpp /Volumes/RAMDisk/buildbot-root/x86_64-darwin/build/projects/cross-project-tests/debuginfo-tests/llgdb-tests/Output/nested-struct.cpp.tmp.o
# .---command stderr------------
# | 'PYTHON_EXEC_PATH=/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/bin/python3.9': command not found
# `-----------------------------
# error: command failed with exit status: 127

--

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


```

</details>

https://github.com/llvm/llvm-project/pull/203138