Enable share-generics in release bundle builds (#63432)

Cargo joins `build.rustflags` across config sources, so the base
`symbol-mangling-version=v0` and `tokio_unstable` flags still apply
after the newly added config.

With sccache disabled, `cargo build --release -p zed` on macOS: 
Before: zed binary 420677880 bytes
After: zed binary 375109048 bytes (-10.8%, -45.6MB)

After `strip -x` of both
(https://github.com/zed-industries/zed/pull/63429):
313879024 -> 307727432 bytes (-2.0%, -6.2MB)
The rest of the delta is the smaller symbol table. `drop_glue`
instantiations: 101778 -> 65041.

Release Notes:

- N/A

script/bundle-linux

@@ -83,7 +83,7 @@ if [[ "$(uname -m)" == "aarch64" ]]; then
 else
     export RUSTFLAGS="${RUSTFLAGS:-} -C link-args=-Wl,--disable-new-dtags,-rpath,\$ORIGIN/../lib"
 fi
-cargo build --release --target "${target_triple}" --package zed --package cli
+cargo --config .cargo/bundle-config.toml build --release --target "${target_triple}" --package zed --package cli
 # Build remote_server in separate invocation to prevent feature unification from other crates
 # from influencing dynamic libraries required by it.
 if [[ "$remote_server_triple" == "$musl_triple" ]]; then
@@ -91,7 +91,7 @@ if [[ "$remote_server_triple" == "$musl_triple" ]]; then
     musl_cc_var="CC_$(echo "$remote_server_triple" | tr '-' '_')"
     export "$musl_cc_var"=musl-gcc
 fi
-cargo build --release --target "${remote_server_triple}" --package remote_server
+cargo --config .cargo/bundle-config.toml build --release --target "${remote_server_triple}" --package remote_server
 
 # Upload debug info to sentry.io
 if ! command -v sentry-cli >/dev/null 2>&1; then