Re-remove excess / for installing SYMLINKS

[lwhsu]

Summary

This excess / was introduced in r280129, and fixed in r295230, but got
re-introduced while merging another branch in r298107.

Test Plan

Do make -DNO_ROOT packagesystem as an unprivileged user in src/release,
and:

1. Check type=link lines in release/dist/METALOG, old:

./base//usr/bin/cpio type=link mode=0755 link=bsdcpio tags=package=runtime

should become:

./base/usr/bin/cpio type=link mode=0755 link=bsdcpio tags=package=runtime

2. Check base.txz contains symbolic files, such as usr/bin/cpio@ -> bsdcpio