mkosi: Allow setting version-tag option via VERSION_TAG env variable

With https://github.com/systemd/mkosi/pull/630, we can set environment
variables for the build script in mkosi. Let's modify the build script
so we can set the version-tag option via an environment variable of the
same name. The default is the empty string which causes meson to fall
back to the default behavior (git version).

mkosi.build

@@ -92,7 +92,8 @@ if [ ! -f "$BUILDDIR"/build.ninja ] ; then
                 -D man=false \
                 -D "nobody-user=$nobody_user" \
                 -D "nobody-group=$nobody_group" \
-                -D translations=false
+                -D translations=false \
+                -D version-tag="${VERSION_TAG}"
 fi
 
 cd "$BUILDDIR"