Fix Python version for sigstore signing on CentOS 7 builds

The sigstore action requires Python 3.9+ and looks for 'python' (not
'python3'). On CentOS 7 (vfx2021 and vfx2022), the system Python is 3.7.9,
so we use uv to install Python 3.11.

Previously, uv only created a python3.11 symlink in ~/.local/bin, which
caused sigstore to fall back to system Python 3.7.9 and fail.

Changes:
- Extend uv Python setup to both vfx2021 and vfx2022 (both CentOS 7)
- Use 'uv python install --default' to create python, python3, and
  python3.11 symlinks in ~/.local/bin
- Simplify PATH setup to just add ~/.local/bin

This ensures the sigstore action finds Python 3.11 as 'python'.

Signed-off-by: Gary Oberbrunner <[email protected]>

Author: garyo

.github/workflows/build.yml

@@ -199,32 +199,26 @@ jobs:
       # Unfortunately astral-sh/setup-uv action doesn't work on CentOS 7, its GLIBC is too old.
 
       - name: Set up uv manually
-        if: matrix.release_prefix == 'linux-vfx2021'
+        if: matrix.vfx-cy == 2021 || matrix.vfx-cy == 2022
         run: |
           curl -LsSf https://astral.sh/uv/install.sh | sh
           source ~/.local/bin/env
-          echo After sourcing uv env: "$PATH"
-          uv python install --preview 3.11
-
-          # Python is in $(uv python dir)/cpython-*/bin
-          # uv also installs a symlink in $HOME/.local/bin but because
-          # that is only a symlink, it causes problems with pip and venv.
-          # As a workaround, prepend the actual bin dir to $PATH.
-          uv_python_dir="$(uv python dir)"
-          uv_cpython_name="$(/bin/ls -1 $uv_python_dir)"
-          uv_python_path="$uv_python_dir/$uv_cpython_name/bin"
-          PATH="$uv_python_path":$PATH
-          # Prepend to path for future steps (note: don't use $GITHUB_ENV here)
-          echo "$uv_python_path" >> $GITHUB_PATH
+          echo "After sourcing uv env, PATH=$PATH"
+
+          # Use --default to create python/python3 symlinks in ~/.local/bin
+          # This ensures the sigstore action can find 'python' (not just 'python3')
+          uv python install --default 3.11
+
+          # Add ~/.local/bin to PATH for future steps
+          echo "$HOME/.local/bin" >> $GITHUB_PATH
 
       - name: Check python, uv paths
         run: |
-          echo $PATH
-          echo -n 'which python: ' && which python
-          echo -n 'which python3: ' && which python3
-          echo -n 'python version: ' && python --version
-          echo -n 'python3 version: ' && python3 --version
-          echo -n 'which uv: ' && (which uv || echo "No python uv; continuing")
+          echo "PATH=$PATH"
+          echo -n 'which python: ' && which python && python --version
+          echo -n 'which python3: ' && which python3 && python3 --version
+          echo -n 'python symlink target: ' && (readlink -f $(which python) || echo "not a symlink")
+          echo -n 'which uv: ' && (which uv || echo "No uv; continuing")
 
       - name: Install Conan
         id: conan