setup-apt-repo: Install gnupg as part of installation.
setup-apt-repo: Install gnupg as part of installation.

Apparently, on Debian stretch, the gnupg package isn't installed by
default, which means that our `apt-key add` commands were failing with
these errors on an ultra-minimal Debian installation:

+ apt-key add ./scripts/setup/packagecloud.asc
E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation
+ apt-key add ./scripts/setup/pgroonga-debian.asc
E: gnupg, gnupg2 and gnupg1 do not seem to be installed, but one of them is required for this operation

Fixes #10480.

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