ci: patch nginx Makefile CFLAGS and LINK in place

Edit the configure-generated assignments instead of appending
CFLAGS += so compile and link both get sanitizer flags, and fail
if CFLAGS is missing -fsanitize=.

Author: lynch1981

.github/workflows/test-nginx.yaml

@@ -88,11 +88,13 @@ jobs:
             --add-module="${GITHUB_WORKSPACE}" \
             --with-http_ssl_module \
             || { echo "::error::nginx configure failed"; cat objs/autoconf.err; exit 1; }
-          printf 'CFLAGS += %s\nLINK = %s %s\n' \
-            "${{ matrix.cc_sanitize }}" \
-            "${CC}" \
-            "${{ matrix.ld_sanitize }}" \
-            >> objs/Makefile
+          sed -i \
+            -e "s|^CFLAGS *=|CFLAGS = ${{ matrix.cc_sanitize }} |" \
+            -e "s|^LINK *=.*|LINK = ${CC} ${{ matrix.ld_sanitize }}|" \
+            objs/Makefile
+          grep -E '^(CFLAGS|LINK)[[:space:]]*=' objs/Makefile
+          grep -q -- '-fsanitize=' <<<"$(grep '^CFLAGS[[:space:]]*=' objs/Makefile)" \
+            || { echo "::error::CFLAGS missing sanitizer flags"; exit 1; }
           make -j"$(nproc)"
           ./objs/nginx -V