Support tag specific environment variables

This makes it possible to specify an environment variable for a tag
that will be used during builds. e.g.

koji edit-tag -x rpm.env.CC=clang

docs/source/using_the_koji_build_system.rst

@@ -395,6 +395,15 @@ environment follows:
   `--old-chroot` option is appended to any mock call. If it is not set,
   mock's default behavior is used.
 
+You may also specify per-tag environment variables for mock to use.
+For example, to set the CC environment variable to clang, you could
+do:
+
+::
+
+    koji edit-tag dnf-fedora-tag -x rpm.env.CC=clang
+
+
 Using Koji to control tasks
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^