ci: fix go-1.13 installation

For some reason the official way of getting older Go versions does not
work, this patch makes the CI downgrade the package on a package manager
level

Signed-off-by: Moritz Poldrack <git@moritz.sh>
Tested-by: builds.sr.ht <builds@sr.ht>
Acked-by: Robin Jarry <robin@jarry.cc>

.builds/alpine-edge.yml

@@ -1,4 +1,3 @@
----
 image: alpine/edge
 packages:
   - go
@@ -25,7 +24,8 @@ tasks:
       cd aerc
       go test ./...
   - ancient-go-version: |
-      go install golang.org/dl/go1.13@latest
-      ~/go/bin/go1.13 download
+      curl -O https://dl-cdn.alpinelinux.org/alpine/v3.11/community/x86_64/go-1.13.13-r0.apk
+      sudo apk add ./go-1.13.13-r0.apk
       cd aerc
-      make GO=~/go/bin/go1.13
+      make clean
+      make