Enable wasm_js backend on CI where needed

This enables it through the `RUSTFLAGS` environment vairable, doing
so for the `wasm32-unknown-unknown` target only.

This is to address:

    error: The wasm32-unknown-unknown targets are not supported by default; you may need to enable the "wasm_js" configuration flag. Note that enabling the `wasm_js` feature flag alone is insufficient. For more information see: https://docs.rs/getrandom/0.3.3/#webassembly-support
       --> /home/runner/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/getrandom-0.3.3/src/backends.rs:168:9
        |
    168 | /         compile_error!(concat!(
    169 | |             "The wasm32-unknown-unknown targets are not supported by default; \
    170 | |             you may need to enable the \"wasm_js\" configuration flag. Note \
    171 | |             that enabling the `wasm_js` feature flag alone is insufficient. \
    172 | |             For more information see: \
    173 | |             https://docs.rs/getrandom/", env!("CARGO_PKG_VERSION"), "/#webassembly-support"
    174 | |         ));

However, it is not necessarily the best way to do it. If we want
the `wasm` feature, which is intentionally not a default feature
even on WASM targets, to be sufficient to enable this behavior,
then it may be necessary to set this up in the build configuration.
The documentation at https://docs.rs/getrandom/0.3.3/getrandom
suggests doing something like:

    [target.wasm32-unknown-unknown]
    rustflags = ['--cfg', 'getrandom_backend="wasm_js"']

However, even assuming we can justifiably and effective do this for
`gix-diff`, which is a library crate, it should still only be done
when the `wasm` feature of `gix-diff` is enabled. That seems like
it might require a `build.rs` file. For now, this change just tests
if enabling that backend is sufficient to let the build succeed.

For more on backends including the `wasm_js` backend, see:
https://docs.rs/getrandom/0.3.3/getrandom/#opt-in-backends

Author: EliahKagan

.github/workflows/ci.yml

@@ -457,6 +457,9 @@ jobs:
           for feature in progress parallel io-pipe crc32 zlib cache-efficiency-debug; do
             cargo build -p gix-features --features "$feature" --target "$TARGET"
           done
+      - name: enable wasm_js backend
+        if: matrix.target == 'wasm32-unknown-unknown'
+        run: echo RUSTFLAGS='--cfg getrandom_backend="wasm_js"' >> "$GITHUB_ENV"
       - name: crates with 'wasm' feature
         run: |
           set -x