provision: Fix buggy management of apt_dependencies_hash.
provision: Fix buggy management of apt_dependencies_hash.

Apparently, we were incorrectly appending each new hash onto the end
of the file, basically resulting in every run of provision being
treated as a miss for this cache.

Fixing this saves about 4s (over 1/3) of the no-op provision time.

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