Saw this transient failure on panther-paladin in CQ:
https://luci-milo.appspot.com/buildbot/chromeos/panther-paladin/17693

> libmojo-395517-r9: sed -e "s:@LIB@:lib:g" -e "s:@BSLOT@:395517:g" /build/panther/tmp/portage/chromeos-base/libmojo-395517-r9/work/libmojo-395517/libmojo.pc.in > /build/panther/tmp/portage/chromeos-base/libmojo-395517-r9/work/libmojo-395517/libmojo-395517.pc
> libmojo-395517-r9: cp: cannot create regular file '/build/panther/tmp/portage/chromeos-base/libmojo-395517-r9/image//usr/src/libmojo-395517/mojo/pylib/mojom/generate/data.pyc': File exists
> libmojo-395517-r9: cp: cannot create regular file '/build/panther/tmp/portage/chromeos-base/libmojo-395517-r9/image//usr/src/libmojo-395517/mojo/pylib/mojom/generate/data.py': File exists
> libmojo-395517-r9: cp: cannot create directory '/build/panther/tmp/portage/chromeos-base/libmojo-395517-r9/image//usr/src/libmojo-395517/mojo/pylib/mojom_tests': File exists