cargo: set MSRV to 1.85

I believe the current stable version of Debian packages 1.85 rustc. So
if the next release of ripgrep uses a higher MSRV, then I think Debian
won't be able to package it.

It also turned out that I wasn't using anything from beyond Rust 1.85
anyway.

It's likely that I could make use of let-chains in various places, but I
don't think it's worth combing through the code to switch to them at
this point.

.github/workflows/ci.yml

@@ -53,7 +53,7 @@ jobs:
         include:
         - build: pinned
           os: ubuntu-latest
-          rust: 1.88.0
+          rust: 1.85.0
         - build: stable
           os: ubuntu-latest
           rust: stable