Add `xcodebuild -runFirstLaunch` fallback for Metal toolchain installation documentation (#51631)

Joey Carpinelli created 

When setting up on macOS 26, `xcodebuild -downloadComponent
MetalToolchain` can fail if Xcode hasn't been initialized yet. Running
`xcodebuild -runFirstLaunch` first resolves this.

Release Notes:

- N/A

Change summary

docs/src/development/macos.md | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

Detailed changes

docs/src/development/macos.md 🔗 

@@ -118,7 +118,8 @@ xcrun: error: unable to find utility "metal", not a developer tool or in PATH
 
 Try `sudo xcode-select --switch /Applications/Xcode.app/Contents/Developer`
 
-If you're on macOS 26, try `xcodebuild -downloadComponent MetalToolchain`
+If you're on macOS 26, try `xcodebuild -downloadComponent MetalToolchain`.
+If that command fails, run `xcodebuild -runFirstLaunch` and try downloading the toolchain again.
 
 ### Cargo errors claiming that a dependency is using unstable features