Use rockylinux8 instead of centos7 in BCR presubmit (#33)

.bcr/presubmit.yml

diff --git a/.bcr/presubmit.yml b/.bcr/presubmit.yml
index aec7695..f81cd02 100644
--- a/.bcr/presubmit.yml
+++ b/.bcr/presubmit.yml
@@ -2,7 +2,7 @@
   module_path: tests/bcr
   matrix:
     platform:
-      - centos7
+      - rockylinux8
       - debian10
       - ubuntu2004
       - macos