ignore: Everything in .ssh/ except config

This contradicts a bit what I wanted in

1497fe3ed4 ("meta:install: Link all files in .ssh/"):
> This way potential new files in `.ssh/` do not have to be tracked
> manually.

as now, even though they do not have to be tracked manually in the
install.conf.yaml, that is the case for the .gitignore

But the consequences of private keys being pushed by accident (even
though not the entire folder is symlinked) are far greater than
potentially missing a symlink, making this a sensible step.
(And the overhead of adding one file is much smaller in the .gitignore)

.gitignore

@@ -30,3 +30,7 @@
 
 # Ignore symlink to meta/envrc in repo root (created in installation)
 /.envrc
+
+# Do not commit any files by accident that might land in there
+/.ssh
+!/.ssh/config