tf-a-windows-builder: migrate builder to bash

We now recommend that TF-A be built in a UNIX environment due to
portability issues in our Makefiles. Migrate the build step in CI to
leverage the bash shell provided by MSYS2.

Change-Id: I42be6c85b9fc8706bbeb19ac3bab3bd0fe4e7db7
Signed-off-by: Harrison Mutai <harrison.mutai@arm.com>
diff --git a/tf-a-windows-builder.yaml b/tf-a-windows-builder.yaml
index 040bf36..2751104 100644
--- a/tf-a-windows-builder.yaml
+++ b/tf-a-windows-builder.yaml
@@ -26,8 +26,9 @@
           values:
             - tf-a-windows
     builders:
-      - batch: |
-          make CROSS_COMPILE=aarch64-none-elf- PLAT=fvp DEBUG=%DEBUG% V=1 -j all
+      - shell:  |
+          #!C:\msys64\usr\bin\env.exe OSTYPE="msys" bash -x
+          make -j $(nproc) CROSS_COMPILE=aarch64-none-elf- PLAT=fvp DEBUG=$DEBUG V=1 all
     parameters:
       - string:
           name: TF_GERRIT_PROJECT