There is no AGENT_TOOLSDIRECTORY on GitHub images (actions#21)

This change reflects what the Windows installer already does:  https://github.com/mjpieters/python-versions/blob/c0e7ef2c3c5b8e3dce50c0caceaafb4a5266a36c/installers/win-setup-template.ps1#L82-L87

Author: mjpieters

installers/nix-setup-template.sh

@@ -9,7 +9,14 @@ PYTHON_MAJOR_DOT_MINOR=python$MAJOR_VERSION.$MINOR_VERSION
 PYTHON_MAJORMINOR=python$MAJOR_VERSION$MINOR_VERSION
 PYTHON_FULL_VERSION=$MAJOR_VERSION.$MINOR_VERSION.$BUILD_VERSION
 
-PYTHON_TOOLCACHE_PATH=$AGENT_TOOLSDIRECTORY/Python
+if [ -z ${AGENT_TOOLSDIRECTORY+x} ]; then
+    # No AGENT_TOOLSDIRECTORY on GitHub images
+    TOOLCACHE_ROOT=$RUNNER_TOOL_CACHE
+else
+    TOOLCACHE_ROOT=$AGENT_TOOLSDIRECTORY
+fi
+
+PYTHON_TOOLCACHE_PATH=$TOOLCACHE_ROOT/Python
 PYTHON_TOOLCACHE_VERSION_PATH=$PYTHON_TOOLCACHE_PATH/$PYTHON_FULL_VERSION
 PYTHON_TOOLCACHE_VERSION_ARCH_PATH=$PYTHON_TOOLCACHE_VERSION_PATH/x64
 
@@ -47,4 +54,4 @@ echo "Upgrading PIP..."
 ./python -m pip install --ignore-installed pip
 
 echo "Create complete file"
-touch $PYTHON_TOOLCACHE_VERSION_PATH/x64.complete
+touch $PYTHON_TOOLCACHE_VERSION_PATH/x64.complete