fix(views/kit): communicate :unsign in the tvl-kit URL directly

Instead of prepending :unsign to all URLs in josh-proxy, and for all
calls to filteredGitPush, explicitly use it only in the filter we use
for the `export-kit` extraStep.

This means, people cloning tvl-kit via

> https://code.tvl.fyi/depot.git:workspace=views/kit.git

now need to update the URL to point to

> https://code.tvl.fyi/depot.git:unsign:workspace=views/kit.git

instead.

git@github.com:tvlfyi/kit.git will keep the same hashes, as it's updated
to export the unsigned workspace view of it.

This is less invasive than dooming every josh workspace to have to strip
signatures.

Change-Id: I6de05182fad4c3695081388c3bbf37306521d255
Reviewed-on: https://cl.tvl.fyi/c/depot/+/8369
Autosubmit: flokli <flokli@flokli.de>
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

tools/releases/default.nix

@@ -19,8 +19,8 @@ in
       set -e
       export PATH="${makeBinPath [ pkgs.git depot.third_party.josh ]}:$PATH"
 
-      echo 'Filtering depot through :unsign ${filter}'
-      josh-filter ':unsign ${filter}'
+      echo 'Filtering depot through ${filter}'
+      josh-filter '${filter}'
 
       echo 'Fetching remote to check if a push is needed'
       git fetch '${remote}' '${ref}'