ci: configure GoReleaser to mark prereleases as such

For some reason this is not default behaviour.

Signed-off-by: Paul Jolly <paul@myitcv.io>
Change-Id: Ibfcc3cc44bc778b16cdf20d0ad205a0616a207a1
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/528610
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
diff --git a/.goreleaser.yml b/.goreleaser.yml
index ed97b51..64ad84f 100644
--- a/.goreleaser.yml
+++ b/.goreleaser.yml
@@ -32,6 +32,7 @@
 
 release:
   disable: false
+  prerelease: auto
 
 checksum:
   name_template: 'checksums.txt'