Handle spaces in environment variables.

The current setup.sh.in doesn't protect bash env variable dereferences against spaces in the PATH environment, which can be found in the Ubuntu bash environment on a modern Win10 system. This fix protects them, and with it, the build of externals and the release works for 21.2.

Note this commit is submitted against 1.0 rather than master. Happy to figure out how to move the commit to master if that is required.