Handle pinned sha versions of Python in the install_requirements action

Author: Ware, Joseph (DLSLtd,RAL,LSCI)

.github/actions/install_requirements/action.yml

@@ -15,7 +15,7 @@ runs:
       run: |
         PYTHON_VERSION="${{ inputs.python-version }}"
         if [ $PYTHON_VERSION == "dev" ]; then
-          PYTHON_VERSION=$(sed -n "s/ARG PYTHON_VERSION=//p" Dockerfile)
+          PYTHON_VERSION=$(sed -Ene "s/ARG PYTHON_VERSION=([0-9\.]+).*/\1/p" Dockerfile)
         fi
         echo "PYTHON_VERSION=$PYTHON_VERSION" >> "$GITHUB_ENV"
       shell: bash