fix: make noarch-unix and noarch-win work

[wolfv]

This could/should fix #742

[wolfv]

This now prints:

     Running `target/debug/rattler-build build --render-only --recipe ./examples/variant --target-platform=win-64`

 ╭─ Finding outputs from recipe
 │ Found 2 variants
 │ Build variant: rattler-build-demo-1-win_4616a5c_0
 │ 
 │ ╭─────────────────┬─────────╮
 │ │ Variant         ┆ Version │
 │ ╞═════════════════╪═════════╡
 │ │ target_platform ┆ noarch  │
 │ ╰─────────────────┴─────────╯
 │ Build variant: rattler-build-demo-subpackage-1-win_b28fc4d_0
 │ 
 │ ╭────────────────────┬─────────────────╮
 │ │ Variant            ┆ Version         │
 │ ╞════════════════════╪═════════════════╡
 │ │ rattler-build-demo ┆ 1 win_4616a5c_0 │
 │ │ target_platform    ┆ noarch          │
 │ ╰────────────────────┴─────────────────╯
 │
 ╰─────────────────── (took 0 seconds)

And

    Finished dev [unoptimized + debuginfo] target(s) in 0.18s
     Running `target/debug/rattler-build build --render-only --recipe ./examples/variant --target-platform=osx-arm64`
 ╭─ Finding outputs from recipe
 │ Found 2 variants
 │ Build variant: rattler-build-demo-1-unix_4616a5c_0
 │ 
 │ ╭─────────────────┬─────────╮
 │ │ Variant         ┆ Version │
 │ ╞═════════════════╪═════════╡
 │ │ target_platform ┆ noarch  │
 │ ╰─────────────────┴─────────╯
 │ Build variant: rattler-build-demo-subpackage-1-unix_2233755_0
 │ 
 │ ╭────────────────────┬──────────────────╮
 │ │ Variant            ┆ Version          │
 │ ╞════════════════════╪══════════════════╡
 │ │ rattler-build-demo ┆ 1 unix_4616a5c_0 │
 │ │ target_platform    ┆ noarch           │
 │ ╰────────────────────┴──────────────────╯
 │
 ╰─────────────────── (took 0 seconds)

[wolfv]

Hmm the tests reveal that there is still a problem lingering ... will come back to this.