Connect the `naga/msl-out` feature with `vello_shaders/msl`. (#583)

The `msl-out` feature of `naga` isn't required unless specifically
requested.

crates/shaders/Cargo.toml

diff --git a/crates/shaders/Cargo.toml b/crates/shaders/Cargo.toml
index 96af358..76462bf 100644
--- a/crates/shaders/Cargo.toml
+++ b/crates/shaders/Cargo.toml
@@ -20,7 +20,7 @@
 
 # Target shading language variants of the vello shaders to link into the library.
 wgsl = []
-msl = []
+msl = ["naga?/msl-out"]
 
 # Enable the CPU versions of the shaders
 cpu = ["dep:bytemuck", "dep:vello_encoding"]
@@ -30,10 +30,10 @@
 
 [dependencies]
 bytemuck = { workspace = true, optional = true }
-naga = { version = "0.20.0", features = ["wgsl-in", "msl-out",], optional = true }
+naga = { version = "0.20.0", features = ["wgsl-in"], optional = true }
 thiserror = { version = "1.0.60", optional = true }
 vello_encoding = { workspace = true, optional = true }
 
 [build-dependencies]
-naga = { version = "0.20.0",  features = ["wgsl-in", "msl-out",] }
+naga = { version = "0.20.0",  features = ["wgsl-in"] }
 thiserror = "1.0.60"