Don't set MICROSHIFT_PRERELEASE

For microshift, this will use microshift 4.17.0-rc.3 while the latest
release is currently microshift 4.17.8

Author: cfergeau

ci_microshift.sh

@@ -5,7 +5,7 @@ set -exuo pipefail
 sudo yum install -y make golang
 
 ./shellcheck.sh
-MICROSHIFT_PRERELEASE=yes ./microshift.sh
+./microshift.sh
 
 # Set the zstd compression level to 10 to have faster
 # compression while keeping a reasonable bundle size.