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/option_processor_test.cc b/src/test/cpp/option_processor_test.cc
index 461b425..e9f09cb 100644
--- a/src/test/cpp/option_processor_test.cc
+++ b/src/test/cpp/option_processor_test.cc
@@ -30,7 +30,7 @@
  protected:
   OptionProcessorTest()
       : workspace_(
-            blaze_util::JoinPath(blaze::GetEnv("TEST_TMPDIR"), "testdir")),
+            blaze_util::JoinPath(blaze::GetPathEnv("TEST_TMPDIR"), "testdir")),
         cwd_("cwd"),
         workspace_layout_(new WorkspaceLayout()) {}