quiche: fix build error with --with-ca-fallback

- Fix build error when curl is built with --with-quiche
  and --with-ca-fallback.

- Add --with-ca-fallback to the quiche CI job.

Fixes https://github.com/curl/curl/issues/11850
Closes https://github.com/curl/curl/pull/11847

.github/workflows/quiche-linux.yml

@@ -72,6 +72,7 @@ jobs:
             --enable-debug
             --with-quiche=/home/runner/quiche/target/release
             --with-test-nghttpx="$HOME/nghttpx/bin/nghttpx"
+            --with-ca-fallback
 
     steps:
     - run: |

lib/vquic/curl_quiche.c

@@ -165,7 +165,7 @@ static CURLcode quic_x509_store_setup(struct Curl_cfilter *cf,
       else {
         /* verifying the peer without any CA certificates won't work so
            use openssl's built-in default as fallback */
-        SSL_CTX_set_default_verify_paths(ssl_ctx);
+        SSL_CTX_set_default_verify_paths(ctx->sslctx);
       }
 #endif
     }