Merge bitcoin/bitcoin#25015: test: Use permissions from git in lint-files.py

908fb7e2ec37fe68675d38dbfee4df9f861bb2b5 test: Use permissions from git in `lint-files.py` (laanwj)
48d2e80a7479a44b0ab09e87542c8cb7a8f72223 test: Don't use shell=True in `lint-files.py` (laanwj)

Pull request description:

  Improvements to the `lint-files.py` script:

  - Avoid use of `shell=True`.
  - Check the permissions in git's metadata instead of in the filesystem. This stops the umask or filesystem from interfering. It's also more efficient as it only needs a single call to `git ls-files`.

  (what triggered this change was `File "..." contains a shebang line, but has the file permission 775 instead of the expected executable permission 755.` errors running the script locally).

ACKs for top commit:
  vincenzopalazzo:
    re-tACK https://github.com/bitcoin/bitcoin/commit/908fb7e2ec37fe68675d38dbfee4df9f861bb2b5

Tree-SHA512: 2eaf868c55a9c3508b12658a5b3ac429963fd0551e645332d0ac54be56fefccee95115e4667386df24b46b545593cb0d0bf8c6cecab73f9cb19d37ddf704c614

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