ci: configure GoReleaser to mark prereleases as such

For some reason this is not default behaviour.

Signed-off-by: Paul Jolly <[email protected]>
Change-Id: Ibfcc3cc44bc778b16cdf20d0ad205a0616a207a1
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/528610
Unity-Result: CUEcueckoo <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: myitcv

.goreleaser.yml

@@ -32,6 +32,7 @@ archives:
 
 release:
   disable: false
+  prerelease: auto
 
 checksum:
   name_template: 'checksums.txt'