ci_microshift: Remove PRERELEASE flag for 4.17

4.17 is already released so better to consume whatever available as part
of public release instead from mirror which doesn't get updated once
public release happen.

Author: praveenkumar

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.