GHA: fix locale tests on macOS, extend to verify test 1981

- fix macOS locale tests to clear existing variables.
  (Without this, the system-defined `LC_ALL` takes precedence, and
  the custom envs in CI are ignored.)

- trigger test 1981 issue by setting `LC_TIME` to non-English, on macOS.
  (On Linux it'd require explicitly installing a non-English locale, I
  skipped this for simplicity.)
  ```
  [...]
  -Time: 01/Aug/2025 08:31:43.037103 +0000 UTC[CR][LF]
  +Time: 01/ao%c3%bb/2025 08:31:43.037103 +0000 UTC[CR][LF]
  [...]
  FAIL 1981: '%time output with --write-out' HTTP, HTTP GET
  ```
  Follow-up to 90a7732d46 #21749

Follow-up to 1cc8a5235f #17988
Follow-up to c221c0ee59 #17938

Closes #21753

.github/workflows/linux.yml

@@ -89,7 +89,7 @@ jobs:
           - name: 'libressl krb5'
             image: ubuntu-24.04-arm
             install_packages: libidn2-dev libnghttp2-dev libldap-dev libkrb5-dev
-            install_steps: libressl-c-arm pytest codeset-test
+            install_steps: libressl-c-arm pytest codeset-test1
             LDFLAGS: -Wl,-rpath,/home/runner/libressl/lib
             configure: --with-openssl=/home/runner/libressl --with-gssapi --enable-debug
 
@@ -953,7 +953,7 @@ jobs:
             TFLAGS+=' --buildinfo'  # only test-ci sets this by default, set it manually for test-torture
           fi
           [ -f ~/venv/bin/activate ] && source ~/venv/bin/activate
-          if [[ "${MATRIX_INSTALL_STEPS}" = *'codeset-test'* ]]; then
+          if [[ "${MATRIX_INSTALL_STEPS}" = *'codeset-test1'* ]]; then
             locale || true
             export LC_ALL=C
             export LC_CTYPE=C