chore: use Rust 1.97 cargo warnings (#47)

- [x] I agree to follow the project's [code of
conduct](https://github.com/georust/geo/blob/master/CODE_OF_CONDUCT.md).
~- [ ] I added an entry to `CHANGES.md` if knowledge of this change
could be valuable to users.~
---

Author: nyurik

justfile

@@ -8,12 +8,11 @@ binstall_args := if env('CI', '') != '' {'--no-confirm --no-track --disable-tele
 # location of the coverage output, used by CI
 coverage_lcov := 'target/llvm-cov/lcov.info'
 
-# if running in CI, treat warnings as errors by setting RUSTFLAGS and RUSTDOCFLAGS to '-D warnings' unless they are already set
+# if running in CI, treat warnings as errors by setting CARGO_BUILD_WARNINGS to 'deny' unless it is already set
 # Use `CI=true just ci-test` to run the same tests as in GitHub CI.
-# Use `just env-info` to see the current values of RUSTFLAGS and RUSTDOCFLAGS
+# Use `just env-info` to see the current value of CARGO_BUILD_WARNINGS
 ci_mode := if env('CI', '') != '' {'1'} else {''}
-export RUSTFLAGS := env('RUSTFLAGS', if ci_mode == '1' {'-D warnings'} else {''})
-export RUSTDOCFLAGS := env('RUSTDOCFLAGS', if ci_mode == '1' {'-D warnings'} else {''})
+export CARGO_BUILD_WARNINGS := env('CARGO_BUILD_WARNINGS', if ci_mode == '1' {'deny'} else {'warn'})
 export RUST_BACKTRACE := env('RUST_BACKTRACE', if ci_mode == '1' {'1'} else {'0'})
 
 @_default:
@@ -75,8 +74,7 @@ env-info:
     rustc --version
     cargo --version
     rustup --version
-    @echo "RUSTFLAGS='$RUSTFLAGS'"
-    @echo "RUSTDOCFLAGS='$RUSTDOCFLAGS'"
+    @echo "CARGO_BUILD_WARNINGS='$CARGO_BUILD_WARNINGS'"
     @echo "RUST_BACKTRACE='$RUST_BACKTRACE'"
 
 # Reformat all code `cargo fmt`. If nightly is available, use it for better results
@@ -170,6 +168,7 @@ assert-git-is-clean:
 cargo-install $COMMAND $INSTALL_CMD='' *args='':
     #!/usr/bin/env bash
     set -euo pipefail
+    unset CARGO_BUILD_WARNINGS
     if ! command -v $COMMAND > /dev/null; then
         echo "$COMMAND could not be found. Installing..."
         if ! command -v cargo-binstall > /dev/null; then