GHA: migrate 3 linter jobs to arm64

Also to make them finish as fast or overall faster.

checkdocs/proselint:
before: https://github.com/curl/curl/actions/runs/21255607528/job/61169136666 22s
after: https://github.com/curl/curl/actions/runs/21266680535/job/61207470652?pr=20403 19s

checksrc/spellcheck, linters, REUSE:
before: https://github.com/curl/curl/actions/runs/21255607522/job/61169137076 3m11s
after: https://github.com/curl/curl/actions/runs/21266680282/job/61207475282?pr=20403 2m27s
typos: 13s -> 28s
pytype: 2m24s -> 1m24s

checksrc/misc checks:
before: https://github.com/curl/curl/actions/runs/21255607522/job/61169137176 2m29s
after: https://github.com/curl/curl/actions/runs/21266680282/job/61207475248?pr=20403 2m5s
shellcheck: seems larger and slower on arm64. 33 -> 45MB, 1s -> 3s
badwords: 2m1s -> 1m35s

Closes #20403

.github/workflows/checkdocs.yml

@@ -38,7 +38,7 @@ jobs:
   # config file help: https://github.com/amperser/proselint/
   proselint:
     name: 'proselint'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04-arm
     steps:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:

.github/workflows/checksrc.yml

@@ -43,7 +43,7 @@ jobs:
 
   linters:
     name: 'spellcheck, linters, REUSE'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04-arm
     steps:
       - uses: actions/checkout@8e8c483db84b4bee98b60c0593521ed34d9990e8 # v6.0.1
         with:
@@ -135,7 +135,7 @@ jobs:
 
   miscchecks:
     name: 'misc checks'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-24.04-arm
     timeout-minutes: 5
     steps:
       - name: 'install prereqs'