install: Run git config commands from a known readable cwd.
install: Run git config commands from a known readable cwd.

Fixes this error when running the installer from a directory that
isn’t world-readable:

+ su zulip -c 'git config --global user.email dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com'
fatal: cannot come back to cwd: Permission denied

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

kou/zulip,rht/zulip,punchagan/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,kou/zulip,kou/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,rht/zulip,kou/zulip,rht/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,eeshangarg/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,zulip/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,zulip/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,hackerkid/zulip