Fix build on MacOS

[robo9k]

A recent CI run on MacOS failed with the following error:

ld: in /Users/runner/.rustup/toolchains/1.38.0-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libbacktrace-fd4028660702ce49.rlib(rust.metadata.bin), archive member 'rust.metadata.bin' with length 87689 is not mach-o or llvm bitcode file '/Users/runner/.rustup/toolchains/1.38.0-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib/libbacktrace-fd4028660702ce49.rlib'

The GitHub Actions CI build uses macos-latest. According to https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/.E2.9C.94.20How.20can.20I.20fix.20Rust.201.2E53.2E0.20or.20earlier.20to.20run.20on.20macOS.2012.2E6.3F/near/299263887 the used MSRV of 1.38.0-x86_64-apple-darwin does not work as-is.

Options are:

• not using the latest MacOS version in CI (but users might do this)
• change build to use another linker or thin binaries
• change build to MSRV 1.54.0