[release-branch.go1] src/all.bat: fix banner

««« backport 2f7827571b44
src/all.bat: fix banner
we need to restore %PATH% before calling "dist banner", so that
it could suggest the user to add %GOROOT%/bin to %PATH% if necessary.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6525049

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