Allow download location to be specified

[0xTim]

On Linux, Swiftly downloads the toolchain to install to /tmp. On some instances this is mounted to tmpfs so limited by the amount of RAM available to the machine. If this is small (enough.g. 1GB), Swiftly fails with a confusing error message Error: The operation could not be completed. There isn’t enough space. even if there's plenty of disk space.

Ideally we'd handle this but providing a flag to download the toolchains to would be a good compromise

[cmcgee1024]

@0xTim it appears that FileManager.temporaryDirectory honours the standard TMPDIR environment variable. Can you try doing an installation like this and see if this works for this use case?

TMPDIR=/another/tmp/with/space swiftly install latest

Initially, I was thinking that we could use /var/tmp for downloads, but that's intended for temporary storage that persists between reboots, which is not at all the case here. These are big files, which is what /var is intended to be used. It's just that the tmp underneath doesn't work for this.

[0xTim]

I'll pass this on and see if it fixes it. Will close for now and reopen if it's still an issue.

(Would be great if these were documented!)

[cmcgee1024]

@0xTim I was going to use this issue to add some documentation after hearing back whether this works. I'll raise a draft PR to document this. I think that TMPDIR is a fairly common idiom in Unix world, but it can't hurt to mention it again in the documentation.

[cmcgee1024]

See #308