test-install: Share the pip cache across installs.
test-install: Share the pip cache across installs.

This saves several minutes off the install time.  Sadly pip still
clones Git repos for dependencies that point to them, but for many
others (not all? not sure) it just gets a wheel from the cache.

dhcrzf/zulip,brainwane/zulip,showell/zulip,shubhamdhama/zulip,hackerkid/zulip,hackerkid/zulip,timabbott/zulip,timabbott/zulip,punchagan/zulip,rht/zulip,kou/zulip,brainwane/zulip,hackerkid/zulip,dhcrzf/zulip,showell/zulip,tommyip/zulip,dhcrzf/zulip,shubhamdhama/zulip,showell/zulip,tommyip/zulip,rht/zulip,brainwane/zulip,zulip/zulip,hackerkid/zulip,punchagan/zulip,synicalsyntax/zulip,timabbott/zulip,punchagan/zulip,tommyip/zulip,eeshangarg/zulip,brainwane/zulip,showell/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,jackrzhang/zulip,punchagan/zulip,hackerkid/zulip,timabbott/zulip,rishig/zulip,kou/zulip,jackrzhang/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,rishig/zulip,eeshangarg/zulip,rishig/zulip,rishig/zulip,shubhamdhama/zulip,timabbott/zulip,rht/zulip,timabbott/zulip,jackrzhang/zulip,zulip/zulip,rishig/zulip,showell/zulip,tommyip/zulip,shubhamdhama/zulip,rishig/zulip,andersk/zulip,brainwane/zulip,synicalsyntax/zulip,jackrzhang/zulip,hackerkid/zulip,eeshangarg/zulip,brainwane/zulip,zulip/zulip,eeshangarg/zulip,showell/zulip,kou/zulip,shubhamdhama/zulip,shubhamdhama/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,synicalsyntax/zulip,zulip/zulip,dhcrzf/zulip,zulip/zulip,rishig/zulip,andersk/zulip,andersk/zulip,synicalsyntax/zulip,shubhamdhama/zulip,kou/zulip,kou/zulip,tommyip/zulip,zulip/zulip,dhcrzf/zulip,brainwane/zulip,andersk/zulip,dhcrzf/zulip,timabbott/zulip,jackrzhang/zulip,jackrzhang/zulip,synicalsyntax/zulip,showell/zulip,tommyip/zulip,dhcrzf/zulip,tommyip/zulip,andersk/zulip,jackrzhang/zulip,synicalsyntax/zulip,synicalsyntax/zulip,rht/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip