fix: ensure exec staging dir, preferring user dirs over tmp

[jonaslb]

Fixes #324 . Related to astral-sh/uv#17191.

As a bit extra context for the issue, /tmp is sometimes mounted noexec, as it is I believe considered "security best practice" according to CIS. So probably, most users hit by this are enterprisy. But I suppose there might be some distros out there as well.

This PR handles the issue by:

1. Walking through a list of priorities, starting with runtime dir, cache dir and data-local dir (using directories crate that I added as dependency), and then finally TMPDIR at last as before.
2. For each one, if it exists and we can create a subfolder, it tests if it is possible to execute from there (assuming true on non-unix). This happens by writing a small no-op script.
   • In practice we're going to mostly resolve /run/user/<uid> on Linux, $HOME/Library/Caches on MacOS, and {FOLDERID_LocalAppData} on Windows (as documented by the directories crate).
3. Still uses the tempfile library as before for cleanup on drop

I've tested this with a script that sets up a "fake root":

#!/bin/bash -eux

ROOT=/tmp/axo-staging-check
TMPDIR=$ROOT/tmp
XDG_RUNTIME_DIR=$ROOT/runtime
XDG_CACHE_HOME=$ROOT/cache
XDG_DATA_HOME=$ROOT/data

domount() {
	sudo mkdir -p $1
	sudo mount -t tmpfs -o rw,nosuid,nodev,size=512m,$2 tmpfs $1
}
domount $TMPDIR noexec,mode=777
domount $XDG_RUNTIME_DIR exec,mode=700,uid=$(id -u),gid=$(id -g)
domount $XDG_CACHE_HOME exec,mode=775,uid=$(id -u),gid=$(id -g)
domount $XDG_DATA_HOME exec,mode=775,uid=$(id -u),gid=$(id -g)

# Cargo will use our fake TMPDIR otherwise, filling it
cargo test --no-run
# Now run test
set +e
RUST_BACKTRACE=1 \
TMPDIR=$TMPDIR \
XDG_RUNTIME_DIR=$XDG_RUNTIME_DIR \
XDG_CACHE_HOME=$XDG_CACHE_HOME \
XDG_DATA_HOME=$XDG_DATA_HOME \
cargo test -p axoupdater-cli --test integration test_upgrade -- --nocapture

# Do cleanup
# ls -la $TMPDIR $XDG_RUNTIME_DIR $XDG_CACHE_HOME $XDG_DATA_HOME
sudo umount $TMPDIR $XDG_RUNTIME_DIR $XDG_CACHE_HOME $XDG_DATA_HOME

Although it's not easy to run the tests due to rate limiting from Github. I also had to adapt the cli tests to have a fallback to the runtime dir themselves, as they each individually do the whole "download and execute" dance, which won't work with a noexec tmp either.