Do all CI builds with old rustc using shim crate

Fixes the following error when testing against a compiler older
than 1.64:

    error: failed to select a version for the requirement `hashbrown = "^0.14"`
    candidate versions found which didn't match: 0.13.2, 0.13.1, 0.12.3, ...
    location searched: crates.io index
    required by package `indexmap v2.0.0`
        ... which satisfies dependency `indexmap = "^2"` of package `serde_json v1.0.97`
        ... which satisfies path dependency `serde_json` of package `serde_json_test v0.0.0`

Author: dtolnay

.github/workflows/ci.yml

@@ -52,17 +52,17 @@ jobs:
         with:
           toolchain: ${{matrix.rust}}
           targets: ${{matrix.target}}
-      - run: cargo check
-      - run: cargo check --features float_roundtrip
-      - run: cargo check --features arbitrary_precision
-      - run: cargo check --features raw_value
-      - run: cargo check --features unbounded_depth
+      - run: cargo check --manifest-path tests/crate/Cargo.toml
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --features float_roundtrip
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --features arbitrary_precision
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --features raw_value
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --features unbounded_depth
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,arbitrary_precision
       - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,raw_value
-      - run: cargo check --features preserve_order
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --features serde_json/preserve_order
         if: matrix.rust != '1.56.1'
-      - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,preserve_order
+      - run: cargo check --manifest-path tests/crate/Cargo.toml --no-default-features --features alloc,serde_json/preserve_order
         if: matrix.rust != '1.56.1'
       - name: Build without std
         run: cargo check --manifest-path tests/crate/Cargo.toml --target ${{matrix.target}} --no-default-features --features alloc

tests/crate/Cargo.toml

@@ -16,6 +16,7 @@ default = ["std"]
 std = ["serde_json/std"]
 alloc = ["serde_json/alloc"]
 #preserve_order = ["serde_json/preserve_order"]
+float_roundtrip = ["serde_json/float_roundtrip"]
 arbitrary_precision = ["serde_json/arbitrary_precision"]
 raw_value = ["serde_json/raw_value"]
 unbounded_depth = ["serde_json/unbounded_depth"]