fix CI to use older deps with older toolchain

Older rust has a broken half-implementation of MSRV support, in that `cargo update` (or when no lockfile is present) will grab the lastest package, ignoring its MSRV, yet immediately complain that it can't build said package because its MSRV is too high.

Author: wfraser

Diff for: .github/workflows/cargo-test.yml

@@ -41,6 +41,12 @@ jobs:
       - name: Install MSRV toolchain
         run: rustup install 1.75.0 --profile minimal
 
+      # remove this once our MSRV is >= 1.84, which has the MSRV-aware resolver
+      - name: fix deps for old rustc
+        run: |
+          rustup run 1.75.0 cargo update litemap --precise 0.7.4
+          rustup run 1.75.0 cargo update zerofrom --precise 0.1.5
+
       - name: Run cargo test
         run: rustup run 1.75.0 cargo test --all-features