[flang] Switch from check-all to check-flang

With check-all this builder captures too many generic failures that are
not specific to Flang. We are switching to check-flang for better
granularity.

Differential Revision: https://reviews.llvm.org/D90308
diff --git a/buildbot/osuosl/master/config/builders.py b/buildbot/osuosl/master/config/builders.py
index 6dd235d..2f4d662 100644
--- a/buildbot/osuosl/master/config/builders.py
+++ b/buildbot/osuosl/master/config/builders.py
@@ -1746,6 +1746,7 @@
     'factory' : UnifiedTreeBuilder.getCmakeWithNinjaBuildFactory(
                     clean=True,
                     depends_on_projects=['llvm','mlir','clang','flang'],
+                    checks=['check-flang'],
                     extra_configure_args=[
                         "-DLLVM_TARGETS_TO_BUILD=AArch64",
                         "-DCMAKE_C_COMPILER=/usr/bin/clang-10",