Support rig install 3.6 without extra switch on arm64

[krlmlr]

Current behavior, it took me longer than anticipated to come up with the necessary -a x86_64 switch:

➜  ~ rig resolve oldrel-3
[ERROR] Failed to resolve R version: Unknown version specification: oldrel-3
➜  ~ rig resolve 3.6
3.6.3 NA
➜  ~ rig install 3.6
[INFO] Running `sudo` for adding new R versions. This might need your password.
Password:
[ERROR] Cannot find a download url for R version 3.6.3, arm64
➜  ~ rig install 3.6-x86_64
[INFO] Running `sudo` for adding new R versions. This might need your password.
[ERROR] Failed to resolve R version: Unknown version specification: 3.6-x86_64
➜  ~ rig install 3.6 -a x86_64
[INFO] Running `sudo` for adding new R versions. This might need your password.
[INFO] x86_64-R-3.6.3.pkg is cached at /tmp/rig/x86_64-R-3.6.3.pkg

[gaborcsardi]

It is literally in rig add --help, though:

    # Install arm64 build of R (default on arm64 macs)
    rig add -a arm64 release

    # Install x86_64 build of R (default on x86_64 macs)
    rig add -a x86_64 release

[krlmlr]

Fair enough. Maybe rig resolve should fail instead of printing NA ? Should rig resolve oldrel-3 work?

[gaborcsardi]

If we want oldrel/3 to work, then we might as well, change the default arch for older R versions.