Ignore some other virtual environment directories

Like ".venv" and "venv", ".env" and "env" are common, plus "env"
appears in the example command shown for making a virtual
environment for the purpose of building a release, under some
circumstances when "make release" or "make force_release" fail.

.gitignore

@@ -1,6 +1,8 @@
 *.py[co]
 *.swp
 *~
+.env/
+env/
 .venv/
 venv/
 /*.egg-info