Emit Python version metric events for all builds (#1066)

Previously the metric events describing the chosen Python version were
only emitted when that Python version was installed, and not when it
was being used from the build cache (the common case).

Now the version is emitted for all builds, improving visibility into
the distribution of Python usage, and helping determine the priority
of features like opt-in automatic Python patch updates.

Closes @W-8059668@.

bin/steps/python

@@ -80,6 +80,8 @@ else
   exit 1
 fi
 
+mcount "version.python.${PYTHON_VERSION}"
+
 if [[ "$STACK" != "$CACHED_PYTHON_STACK" ]]; then
     puts-step "Stack has changed from $CACHED_PYTHON_STACK to $STACK, clearing cache"
     rm -fr .heroku/python-stack .heroku/python-version .heroku/python .heroku/vendor .heroku/python .heroku/python-sqlite3-version
@@ -119,8 +121,6 @@ if [ ! "$SKIP_INSTALL" ]; then
     # Prepare destination directory.
     mkdir -p .heroku/python
 
-    mcount "version.python.$PYTHON_VERSION"
-
     if ! curl "${VENDORED_PYTHON}" -s | tar zxv -C .heroku/python &> /dev/null; then
       puts-warn "Requested runtime ($PYTHON_VERSION) is not available for this stack ($STACK)."
       puts-warn "Aborting.  More info: https://devcenter.heroku.com/articles/python-support"