Auto-install hawkeye in ensure-hawkeye-exists.sh (#1644)

- Fixes #1642.
- The `ensure-hawkeye-exists.sh` now actually
  ensures that hawkeye is installed. If it is not
  installed, the script informs that the installation
  uses `curl | sh` and asks the user to confirm
  before proceeding.
- Automated workflows can bypass the prompt
  by invoking the script with the `-y`/`--auto-install`
  option, or by setting the environment variable
  `HAWKEYE_AUTO_INSTALL=1`.
- Export HAWKEYE_AUTO_INSTALL=1  in every
  Git workflow job that runs make check, to
  ensure license/format checks don't stall.

.github/workflows/common.yml

@@ -35,6 +35,8 @@ jobs:
           fetch-depth: 0
 
       - name: Check formatting
+        env:
+          HAWKEYE_AUTO_INSTALL: "1"
         run: |
           ./scripts/install-hawkeye.sh
           make fmt