Fix build on Windows

It seems like the `sed` on Windows is not particularly
smart. It's not actually needed in this place, so I've
removed it's usage and just created an invalid yaml
another way.

lld/test/COFF/vfsoverlay.test

@@ -9,7 +9,7 @@
 # RUN:   | FileCheck %s
 # CHECK: error: {{[Nn]}}o such file or directory
 
-# RUN: sed -e "s|{|bad|g" %t/overlay.yaml > %t/badoverlay.yaml
+# RUN: echo "invalid yaml" > %t/badoverlay.yaml
 # RUN: not lld-link %S/Inputs/hello64.obj /libpath:/noexist /out:%t.exe /entry:main /defaultlib:notstd64 /vfsoverlay:%t/badoverlay.yaml 2>&1 \
 # RUN:   | FileCheck %s --check-prefix=BAD-OVERLAY
 # BAD-OVERLAY: error: Invalid vfs overlay