GH-41088: [CI][Crossbow] Fix GitHub Actions workflow syntax error (#4…

…1091)

### Rationale for this change

We can't use multiple top-level `env:` in workflow. GH-40949 introduced a top-level `env:` by `macros.github_header()`. It broke workflows that already have top-level `env:`.

### What changes are included in this PR?

Omit top-level `env:` key and reuse the top-level `env:` key generated by `macros.github_header()` in workflows.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* GitHub Issue: #41088

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

dev/tasks/python-wheels/github.osx.yml

@@ -17,8 +17,7 @@
 {% import 'macros.jinja' as macros with context %}
 
 {{ macros.github_header() }}
-
-env:
+# env: is generated by macros.github_header()
   ARROW_JEMALLOC: "{{ arrow_jemalloc }}"
   CC: "clang"
   CMAKE_BUILD_TYPE: release

dev/tasks/verify-rc/github.macos.yml

@@ -20,8 +20,7 @@
 {{ macros.github_header() }}
 
 {% set use_conda = use_conda|default(False) %}
-
-env:
+# env: is generated by macros.github_header()
   # Current oldest supported version according to https://endoflife.date/macos
   MACOSX_DEPLOYMENT_TARGET: "10.15"