.builds: switch to Go 1.14rc1 release downloads

On Linux and FreeBSD, this means we no longer need to install Go twice,
since we don't need to bootstrap a Go build. This reduces the "install
go" step by over half a minute, and avoids installing a number of distro
packages.

Debian requires the pkg-config package to be explicit, as it's not part
of the build-essential metapackage, and we need it. It was being pulled
as a dependency from the golang package before.

OpenBSD is nearly the same change, modulo the fact that Go doesn't have
a binary release for it. So instead, we keep installing Go via the
system's package, and we download and build a source archive.

Finally, switch from Debian testing+experimental to simply unstable.
Sway 1.4 finally hit unstable a few days ago, and experimental is
extremely unstable, so this is a step in the right direction. Add a TODO
about going to just testing once sway 1.4 arrives there.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

.builds/openbsd.yml

@@ -5,13 +5,13 @@ packages:
 sources:
  - https://git.sr.ht/~eliasnaur/gio
 environment:
-    PATH: /home/build/bin:/bin:/usr/local/bin:/usr/bin
+ PATH: /home/build/sdk/go/bin:/bin:/usr/local/bin:/usr/bin
 tasks:
- - install_gotip: |
-     go get golang.org/dl/gotip
-     /home/build/go/bin/gotip download
-     mkdir /home/build/bin
-     ln -s /home/build/go/bin/gotip /home/build/bin/go
+ - install_go1_14rc1: |
+     mkdir -p /home/build/sdk
+     curl https://dl.google.com/go/go1.14rc1.src.tar.gz | tar -C /home/build/sdk -xzf -
+     cd /home/build/sdk/go/src
+     ./make.bash
  - test_gio: |
      cd gio
      go test ./...