postgresql@9.6: fix ENV["CI"] usage.

Part of #70493.

Formula/postgresql@9.6.rb

@@ -87,9 +87,12 @@ def install
   end
 
   def post_install
-    return if ENV["CI"]
-
     (var/"log").mkpath
+    postgresql_datadir.mkpath
+
+    # Don't initialize database, it clashes when testing other PostgreSQL versions.
+    return if ENV["HOMEBREW_GITHUB_ACTIONS"]
+
     postgresql_datadir.mkpath
     system "#{bin}/initdb", postgresql_datadir unless pg_version_exists?
   end
@@ -149,7 +152,7 @@ def plist
   end
 
   test do
-    system "#{bin}/initdb", testpath/"test" unless ENV["CI"]
+    system "#{bin}/initdb", testpath/"test" unless ENV["HOMEBREW_GITHUB_ACTIONS"]
     assert_equal pkgshare.to_s, shell_output("#{bin}/pg_config --sharedir").chomp
     assert_equal lib.to_s, shell_output("#{bin}/pg_config --libdir").chomp
     assert_equal lib.to_s, shell_output("#{bin}/pg_config --pkglibdir").chomp