ci: Add a way to skip semver checks

Include the following in the PR body:

    ci: ignore-semver

And semver checks should be skipped.

Author: tgross35

.github/workflows/ci.yaml

@@ -13,6 +13,11 @@ env:
   RUSTDOCFLAGS: -Dwarnings
   RUSTFLAGS: -Dwarnings
   RUST_BACKTRACE: full
+  # Skip semver checks if specified. This implementation is a bit crude but
+  # seems to work.
+  SEMVER_CHECKS: >
+    ${{ contains(github.event.pull_request.bodyText, '\nci: ignore-semver')
+        && 'ignore' || 'check' }}
 
 defaults:
   run:

ci/run-docker.sh

@@ -43,6 +43,7 @@ run() {
         --user "$(id -u)":"$(id -g)" \
         --env LIBC_CI \
         --env LIBC_CI_ZBUILD_STD \
+        --env SEMVER_CHECKS \
         --env RUST_LIBC_UNSTABLE_GNU_FILE_OFFSET_BITS \
         --env RUST_LIBC_UNSTABLE_GNU_TIME_BITS \
         --env CARGO_HOME=/cargo \

ci/verify-build.sh

@@ -113,8 +113,13 @@ test_target() {
     # FIXME(semver): can't pass `--target` to `cargo-semver-checks`
     if [ "$rust" = "stable" ] && [ "$target" = "$host_target" ]; then
         # Run semver checks on the stable channel
-        cargo semver-checks --only-explicit-features \
-            --features std,extra_traits
+        cmd="cargo semver-checks --only-explicit-features --features std,extra_traits"
+
+        if [ "$SEMVER_CHECKS" = "ignore" ]; then
+            $cmd || true
+        else
+            $cmd
+        fi
     fi
 }