circleci: Use CIRCLE_JOB env var for distro in cache filenames.
circleci: Use CIRCLE_JOB env var for distro in cache filenames.

This lets us remove the ugly _dist.txt hack from our CircleCI
configuration.

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