[setup-r] Windows ARM RTools environment variables not correctly set

[andrjohns]

Describe the bug

After using the setup-r action on a windows-11-arm runner, the RTOOLS45_HOME environment variable is set to the x86_64 toolchain location C:/rtools45 and the RTOOLS45_AARCH64_HOME environment variable is unset. This breaks workflows which use these values for toolchain lookups.

To Reproduce

On a github actions run with a windows-11-arm runner I see:

Sys.getenv()[grep("_HOME", names(Sys.getenv()), value = TRUE)]
# R_HOME                  C:/R
# RTOOLS45_HOME           C:\rtools45

Expected behavior

On a local Windows ARM system this returns:

Sys.getenv()[grep("_HOME", names(Sys.getenv()), value = TRUE)]
# R_HOME                  C:/PROGRA~1/R-AARC~1/R-46~1.1
# RTOOLS45_AARCH64_HOME   C:\rtools45-aarch64

Thanks!