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

src/main/cpp/blaze_util_linux.cc

diff --git a/src/main/cpp/blaze_util_linux.cc b/src/main/cpp/blaze_util_linux.cc
index a365ac9..e6e6d64 100644
--- a/src/main/cpp/blaze_util_linux.cc
+++ b/src/main/cpp/blaze_util_linux.cc
@@ -130,7 +130,7 @@
 }
 
 static string Which(const string &executable) {
-  string path(GetEnv("PATH"));
+  string path(GetPathEnv("PATH"));
   if (path.empty()) {
     return "";
   }
@@ -154,7 +154,7 @@
 
 string GetSystemJavabase() {
   // if JAVA_HOME is defined, then use it as default.
-  string javahome = GetEnv("JAVA_HOME");
+  string javahome = GetPathEnv("JAVA_HOME");
   if (!javahome.empty()) {
     string javac = blaze_util::JoinPath(javahome, "bin/javac");
     if (access(javac.c_str(), X_OK) == 0) {