Use nightly Rust instead of myrust

[q231950]

When running make_fat.sh I was seeing the following output as not all libraries had been compiled:

kim@mbp rust-catalyst-example % ./make_fat.sh
Building for iOS X86_64 (Simulator)...
Building for Mac Catalyst X86_64...
Building for Mac Catalyst ARM64...
Building for ARM iOS...
Building Fat Libaries
fatal error: /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/lipo: can't open input file: target/aarch64-apple-ios-macabi/release/libtest1.a (No such file or directory)
Wrote XcodeIntegration/XcodeIntegration/Rust/libtest1_ios.a
fatal error: /Applications/Xcode-beta.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/lipo: can't open input file: target/aarch64-apple-ios/release/libtest1.a (No such file or directory)
Wrote XcodeIntegration/XcodeIntegration/Rust/libtest1_mac.a

This change replaces myrust the standard nightly Rust so that it compiles for everyone. myrust was probably the Rust that contained rust-lang/rust#77484.