fix CI to use older deps with older toolchain

Older rust has a broken half-implementation of MSRV support, in that  (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

.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