fix rustflags for musl targets

[sokra]

What?

napi sets the RUSTFLAGS env var for musl targets: https://github.com/napi-rs/napi-rs/blob/2d8e19d7c626405287b73a52dc91731c3b89c8df/cli/src/api/build.ts#L467-L485
this means our .cargo/config.toml rustflags are not applied.
This fixes that by adding the RUSTFLAGS env var to the build script

Closes PACK-3844

[sokra]

• fix rustflags for musl targets #75599 👈 (View in Graphite)
• canary

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[ijjk]

Failing test suites

Commit: 8b53be8

pnpm test-start-turbo test/e2e/app-dir/rewrite-headers/rewrite-headers.test.ts (turbopack)

• rewrite-headers > middleware rewrite external Prefetch RSC (/hello/vercel) > should have the expected headers
• rewrite-headers > middleware rewrite external RSC (/hello/vercel) > should have the expected headers

Expand output

● rewrite-headers › middleware rewrite external RSC (/hello/vercel) › should have the expected headers

expect(received).toEqual(expected) // deep equality

- Expected  - 1
+ Received  + 1

  Object {
-   "x-nextjs-rewritten-path": null,
+   "x-nextjs-rewritten-path": "/home",
    "x-nextjs-rewritten-query": null,
  }

  404 |         })
  405 |
> 406 |         expect(headers).toEqual(expected)
      |                         ^
  407 |       })
  408 |     }
  409 |   )

  at Object.toEqual (e2e/app-dir/rewrite-headers/rewrite-headers.test.ts:406:25)

● rewrite-headers › middleware rewrite external Prefetch RSC (/hello/vercel) › should have the expected headers

expect(received).toEqual(expected) // deep equality

- Expected  - 1
+ Received  + 1

  Object {
-   "x-nextjs-rewritten-path": null,
+   "x-nextjs-rewritten-path": "/home",
    "x-nextjs-rewritten-query": null,
  }

  404 |         })
  405 |
> 406 |         expect(headers).toEqual(expected)
      |                         ^
  407 |       })
  408 |     }
  409 |   )

  at Object.toEqual (e2e/app-dir/rewrite-headers/rewrite-headers.test.ts:406:25)

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