Bazel client: add and use blaze::GetPathEnv()

Use GetPathEnv() instead of GetEnv() for envvars
with paths.

On Linux/macOS/POSIX, GetPathEnv and GetEnv do the
same.

On Windows, GetPathEnv removes the UNC prefix from
the result, calls AsWindowsPath, then converts
backslashes to forward slashes. (As callers expect
the result.)

Fixes https://github.com/bazelbuild/bazel/issues/7705

Closes #7707.

PiperOrigin-RevId: 238236143
diff --git a/src/test/cpp/rc_options_test.cc b/src/test/cpp/rc_options_test.cc
index daaba7f..418639e 100644
--- a/src/test/cpp/rc_options_test.cc
+++ b/src/test/cpp/rc_options_test.cc
@@ -34,8 +34,7 @@
 class RcOptionsTest : public ::testing::Test {
  protected:
   RcOptionsTest()
-      : test_file_dir_(blaze::GetEnv("TEST_TMPDIR")),
-        workspace_layout_() {}
+      : test_file_dir_(blaze::GetPathEnv("TEST_TMPDIR")), workspace_layout_() {}
 
   const string test_file_dir_;
   const WorkspaceLayout workspace_layout_;