ci: install hex/rebar before deps.get to fix OTP 28 compat

The previous commit bumped CI from OTP 27.2/Elixir 1.18.1 to
OTP 28.5/Elixir 1.19.5. The broad mix cache restore-keys was
restoring a stale ~/.mix cache with hex compiled for the old
OTP, causing beam_load errors on new bs_add instructions.

Adding explicit mix local.hex --force ensures the correct hex
version for the running OTP is installed before deps.get.

.forgejo/workflows/production.yaml

@@ -81,6 +81,11 @@ jobs:
           apt-get update
           apt-get install -y libssl-dev libsnmp-dev snmp-mibs-downloader postgresql-client
 
+      - name: Install Hex and Rebar
+        run: |
+          mix local.hex --force
+          mix local.rebar --force
+
       - name: Install dependencies
         run: mix deps.get