fix(ci): drop invalid git fetch --depth=0 in Nx base resolution

`actions/checkout@v4` already runs with `fetch-depth: 0`, which pulls
every branch and tag — origin/<base_ref> is therefore present locally
and the explicit fetch is redundant. The flag value `--depth=0` is
itself invalid (git requires a positive integer), so the step failed
with `fatal: depth 0 is not a positive number` on every PR run.

.gitea/workflows/ci.yml

@@ -27,8 +27,11 @@ jobs:
       - name: Derive Nx affected base and head
         shell: bash
         run: |
+          # `actions/checkout@v4` with fetch-depth: 0 already pulls every
+          # branch and tag, so origin/<base_ref> is present locally — no
+          # extra `git fetch` is needed (and `--depth=0` is invalid: git
+          # requires a positive integer).
           if [ "${{ github.event_name }}" = "pull_request" ]; then
-            git fetch origin "${{ github.base_ref }}" --depth=0
             echo "NX_BASE=$(git merge-base HEAD origin/${{ github.base_ref }})" >> "$GITHUB_ENV"
           else
             echo "NX_BASE=HEAD~1" >> "$GITHUB_ENV"