Fix x64_64-gnu-llvm-15 CI tests

There were two command chains separated by `&&` in the script, and since `set -e` doesn't exit for chained commands, if the first chain has failed, the command would happily continue forward, ignoring any test failures.

src/ci/docker/host-x86_64/x86_64-gnu-llvm-15/script.sh

@@ -4,30 +4,30 @@ set -ex
 
 # Only run the stage 1 tests on merges, not on PR CI jobs.
 if [[ -z "${PR_CI_JOB}" ]]; then
-     ../x.py --stage 1 test --skip src/tools/tidy && \
+     ../x.py --stage 1 test --skip src/tools/tidy
      # Run the `mir-opt` tests again but this time for a 32-bit target.
      # This enforces that tests using `// EMIT_MIR_FOR_EACH_BIT_WIDTH` have
      # both 32-bit and 64-bit outputs updated by the PR author, before
      # the PR is approved and tested for merging.
      # It will also detect tests lacking `// EMIT_MIR_FOR_EACH_BIT_WIDTH`,
      # despite having different output on 32-bit vs 64-bit targets.
      ../x.py --stage 1 test tests/mir-opt \
-          --host='' --target=i686-unknown-linux-gnu && \
+          --host='' --target=i686-unknown-linux-gnu
      # Run `ui-fulldeps` in `--stage=1`, which actually uses the stage0
      # compiler, and is sensitive to the addition of new flags.
      ../x.py --stage 1 test tests/ui-fulldeps
 fi
 
 # NOTE: intentionally uses all of `x.py`, `x`, and `x.ps1` to make sure they all work on Linux.
-../x.py --stage 2 test --skip src/tools/tidy && \
+../x.py --stage 2 test --skip src/tools/tidy
      # Run the `mir-opt` tests again but this time for a 32-bit target.
      # This enforces that tests using `// EMIT_MIR_FOR_EACH_BIT_WIDTH` have
      # both 32-bit and 64-bit outputs updated by the PR author, before
      # the PR is approved and tested for merging.
      # It will also detect tests lacking `// EMIT_MIR_FOR_EACH_BIT_WIDTH`,
      # despite having different output on 32-bit vs 64-bit targets.
      ../x --stage 2 test tests/mir-opt \
-                       --host='' --target=i686-unknown-linux-gnu && \
+                       --host='' --target=i686-unknown-linux-gnu
      # Run the UI test suite again, but in `--pass=check` mode
      #
      # This is intended to make sure that both `--pass=check` continues to