Add simple retry to rustup commands in rb_sys/mkmf

[ianks]

Starting to see some sporadic network failures in rustup downloads. We should retry in these cases.

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gem/lib/rb_sys/mkmf.rb:266

• Retry logic for the rustup_toolchain_cmd could be simplified using a loop for better readability and maintainability.

	#{rustup_toolchain_cmd} || (sleep 5; #{rustup_toolchain_cmd}) || (sleep 5; #{rustup_toolchain_cmd})

gem/lib/rb_sys/mkmf.rb:267

• Retry logic for the rustup_default_cmd could be simplified using a loop for better readability and maintainability.

	#{rustup_default_cmd} || (sleep 5; #{rustup_default_cmd}) || (sleep 5; #{rustup_default_cmd})

[ianks]

🙄