Set BAZEL_VC for Bazel's integration tests

To work around [test failure](https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/16353#018901e5-b0f1-41be-a6f4-e12878711ad3) on #18608. This PR enables Bazel to detect the latest VC build tools installed, but apparently there is a test case which is failing with the new toolchain. Setting --test_env=BAZEL_VC ensures the integrations tests also uses the older VC build tools for now.

PiperOrigin-RevId: 544037250
Change-Id: I4625da17ff2168acbe63813aa7a01e49e0cb459a

.bazelci/postsubmit.yml

@@ -338,6 +338,7 @@ tasks:
       - "--host_copt=-w"
       - "--test_tag_filters=-no_windows,-slow"
       - "--test_env=JAVA_HOME"
+      - "--test_env=BAZEL_VC"
       - "--test_env=TEST_INSTALL_BASE=$HOME/bazeltest_install_base"
       - "--test_env=TEST_REPOSITORY_HOME=C:/b/bazeltest_external"
     test_targets:

.bazelci/presubmit.yml

@@ -405,6 +405,7 @@ tasks:
       - "--host_copt=-w"
       - "--test_tag_filters=-no_windows,-slow"
       - "--test_env=JAVA_HOME"
+      - "--test_env=BAZEL_VC"
       - "--test_env=TEST_INSTALL_BASE=$HOME/bazeltest_install_base"
       - "--test_env=TEST_REPOSITORY_HOME=C:/b/bazeltest_external"
     test_targets: