ci: pin ring dep to 0.17.12 to fix MSRV

The latest ring version 0.17.13 changed their MSRV from 1.63 to 1.66.
The bdk_esplora client depends on ring.

ring v0.17.12
├── rustls v0.21.12
│   ├── minreq v2.13.2
│   │   ├── esplora-client v0.11.0
│   │   │   ├── bdk_esplora v0.20.1

Author: notmandatory

ci/pin-msrv.sh

@@ -24,3 +24,4 @@ cargo update -p unicode-width --precise "0.1.13"
 cargo update -p native-tls --precise "0.2.13"
 cargo update -p flate2 --precise "1.0.35"
 cargo update -p bzip2-sys --precise "0.1.12"
+cargo update -p ring --precise "0.17.12"