fix(ci): Ensure idempotence of user creation

Previously, re-running `run.sh` in the same container would fail at the useradd step, because the user already exists. Instead, change that step to "create if not exists" semantics to ease interactive debugging of CI failures.

Split out from rust-lang#111891 per request by @jackh726

src/ci/run.sh

@@ -8,7 +8,7 @@ fi
 
 if [ "$NO_CHANGE_USER" = "" ]; then
   if [ "$LOCAL_USER_ID" != "" ]; then
-    useradd --shell /bin/bash -u $LOCAL_USER_ID -o -c "" -m user
+    id -u $LOCAL_USER_ID &>/dev/null || useradd --shell /bin/bash -u $LOCAL_USER_ID -o -c "" -m user
     export HOME=/home/user
     unset LOCAL_USER_ID