Fix NotADirectoryError in tools/pkg/build.py salt_onedir

Commit 9c268db added a `git add -f salt/_version.txt` invocation
inside `salt_onedir` with `cwd=str(salt_archive)`, but `salt_archive`
is a path to a `.tar.gz` file, not a directory.  Python's subprocess
implementation calls `chdir()` on `cwd` and raises:

    NotADirectoryError: [Errno 20] Not a directory:
    '/.../salt-3008.0+NNN.gSHA.tar.gz'

This blocks the Build Salt Onedir job on Linux and macOS for every
master PR.  The Windows path takes a different branch
(`pkg\\windows\\install_salt.cmd`) and is unaffected.

The `salt_onedir` step runs in the salt source checkout (the CI
working tree), which is a real git repository containing
`salt/_version.txt`, so dropping the `cwd=` argument lets the
subprocess inherit that directory and the `git add` works as the
original commit intended.

Reproducer:

    import subprocess
    subprocess.run(['echo', 'x'], cwd='/tmp/anything.tar.gz')
    # NotADirectoryError: [Errno 20] Not a directory: '/tmp/anything.tar.gz'

Fixes #69461
Refs 9c268db

Author: dwoz

changelog/69461.fixed.md

@@ -0,0 +1,4 @@
+Fix ``NotADirectoryError`` in ``tools/pkg/build.py`` ``salt_onedir`` by no
+longer passing the source tarball path as the ``cwd=`` of the
+``git add -f salt/_version.txt`` subprocess; the command now runs in the
+salt source checkout (a real git working tree) as intended.