clean-npm-cache: Use islink to check node_modules.
clean-npm-cache: Use islink to check node_modules.

Very old Zulip deployments might have node_modules be an actual
directory, which we should ignore for the purposes of
garbage-collection.

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