GHA: start using ARM Linux runners

- GHA/linux: allow per-job runner image override.
- GHA/linux: add an arm version of an existing job.
  Add workaround for broken `PATH` in the arm runner image.
- GHA/non-native: add CPU arch to job name where missing.
- GHA/checkdocs: switch a linter job to arm.

Performance looks a little bit better than Intel
(presumably with lower power consumption).

Test jobs, with openssl, tests, pytests, examples:
- arm:
  - https://github.com/curl/curl/actions/runs/12816430794/job/35737374521 4m7s
  - https://github.com/curl/curl/actions/runs/12816201136/job/35736615144 4m5s
- Intel:
  - https://github.com/curl/curl/actions/runs/12816430794/job/35737374118 4m32s
  - https://github.com/curl/curl/actions/runs/12816201136/job/35736614764 4m13s

Ref: https://github.blog/changelog/2025-01-16-linux-arm64-hosted-runners-now-available-for-free-in-public-repositories-public-preview/

Closes #16023

.github/workflows/non-native.yml

@@ -37,7 +37,7 @@ permissions: {}
 
 jobs:
   netbsd:
-    name: 'NetBSD (CM, openssl, clang)'
+    name: 'NetBSD (CM, openssl, clang, ${{ matrix.arch }})'
     runs-on: ubuntu-latest
     timeout-minutes: 10
     strategy:
@@ -77,7 +77,7 @@ jobs:
             echo '::endgroup::'
 
   openbsd:
-    name: 'OpenBSD (CM, libressl, clang)'
+    name: 'OpenBSD (CM, libressl, clang, ${{ matrix.arch }})'
     runs-on: ubuntu-latest
     timeout-minutes: 10
     strategy: