CI: enable stack protection for -O3 and experimental builds

Because these are a *developer* builds, we don't get
-fstack-protector!  So add it here in both places.

This would have caught the bug found in the next commit.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

.github/workflows/ci.yaml

@@ -18,13 +18,14 @@ jobs:
             TEST_CMD: "make"
           - CFG: "make-O3-check"
             TEST_CMD: "make check-source check-units installcheck check-gen-updated"
-            COPTFLAGS: "-O3"
+            COPTFLAGS: "-O3 -fstack-protector-strong"
           - CFG: "make-32-bit-nodev-check"
             ARCH: 32
             TEST_CMD: "make check-source check-units installcheck"
             DEVELOPER: 0
           - CFG: "make-EXPERIMENTAL-check"
             TEST_CMD: "make check-source check-units installcheck check-gen-updated"
+            COPTFLAGS: "-Og -fstack-protector-strong"
             EXPERIMENTAL_FEATURES: 1
     steps:
       - name: Checkout