Don't build all release artifacts on every pull request commit.
Don't build all release artifacts on every pull request commit.

Github actions resource allowance is limited, so skip building all
artifacts in pull reqeuests. The checks and tests shall be sufficient
for now.

Signed-off-by: Henner Zeller <259ad50fabbfd39355f36ecd6c0afca541ea66ef@acm.org>

chipsalliance/verible,chipsalliance/verible,chipsalliance/verible,chipsalliance/verible,chipsalliance/verible