tools/minify-js: Avoid shelling out for mkdir, cp.
tools/minify-js: Avoid shelling out for mkdir, cp.

Signed-off-by: Anders Kaseorg <087231934ba8fcf78bc142ce37d193b895ac03c4@mit.edu>

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