ci: no debug info and two jobs at most, so the test link fits in the runner's memory

The test job's linker was killed with signal 9 twice (run 705): cargo test links
several test binaries at once, each with full debug info. CARGO_PROFILE_DEV_DEBUG=0
and CARGO_BUILD_JOBS=2 keep the link within the job's memory; the tests are the
same.

.gitea/workflows/test.yml

@@ -11,6 +11,12 @@ jobs:
   rust:
     name: cargo test
     runs-on: ubuntu-latest
+    # The job ran out of memory linking the test binaries (ld killed with
+    # signal 9, run 705 twice): no debug info, and at most two links at once.
+    # What is tested does not change.
+    env:
+      CARGO_PROFILE_DEV_DEBUG: "0"
+      CARGO_BUILD_JOBS: "2"
     steps:
       - uses: actions/checkout@v4
       # rust-toolchain.toml pins the channel (with clippy); rustup installs it on