setup: fix a possible NameError on rust build
File "setup.py", line 975, in rustbuild
"command: %r, environment: %r" % (self.rustsrcdir, cmd, env))
NameError: global name 'cmd' is not defined
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a