On Windows, build ``bazel.exe`` instead of just ``bazel``.

Needed for #276.

--
MOS_MIGRATED_REVID=114693711
diff --git a/scripts/bootstrap/buildenv.sh b/scripts/bootstrap/buildenv.sh
index c2b30fa..63d0897 100755
--- a/scripts/bootstrap/buildenv.sh
+++ b/scripts/bootstrap/buildenv.sh
@@ -36,6 +36,13 @@
   MACHINE_IS_ARM='yes'
 fi
 
+# Extension for executables.
+EXE_EXT=""
+case "${PLATFORM}" in
+msys*|mingw*)
+  EXE_EXT=".exe"
+esac
+
 ATEXIT_=""
 function atexit() {
   ATEXIT_="$1; ${ATEXIT_}"