ci: switch build to GitHub Actions

ci/cargo-out-dir

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+# Finds Cargo's `OUT_DIR` directory from the most recent build.
+#
+# This requires one parameter corresponding to the target directory
+# to search for the build output.
+
+if [ $# != 1 ]; then
+  echo "Usage: $(basename "$0") <target-dir>" >&2
+  exit 2
+fi
+
+# This works by finding the most recent stamp file, which is produced by
+# every ripgrep build.
+target_dir="$1"
+find "$target_dir" -name ripgrep-stamp -print0 \
+  | xargs -0 ls -t \
+  | head -n1 \
+  | xargs dirname