Conditionally use nightly features

This allows us to build on other OSs
4 jobs for build-on-linux in 1 minute and 5 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #18145
build:linux

00:00:14

passed #18146
build:redox

00:00:14

 
  Test
failed #18147
test:linux

00:00:14

passed #18148
test:redox

00:00:50

 
Name Stage Failure
failed
test:linux Test
    tests::getgid
tests::getns
tests::getuid
tests::lseek

test result: FAILED. 0 passed; 16 failed; 0 ignored; 0 measured; 0 filtered out

error: test failed, to rerun pass '--lib'
ERROR: Job failed: exit code 1