Disable MSYS/MSYS2 path conversion, which interferes with Bazel.

Change-Id: I0a0966667baf39310eba885078b99200612e014e
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62790
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>

.github/bazel.sh

@@ -1,6 +1,10 @@
 #!/bin/bash
 set -eux
 
+# Disable MSYS/MSYS2 path conversion, which interferes with Bazel.
+export MSYS_NO_PATHCONV='1'
+export MSYS2_ARG_CONV_EXCL='*'
+
 for compilation_mode in dbg opt
 do
   bazel clean