move the test buckets dir into the canonicalized temp dir

Previously we were using the XDG data dir to avoid symlinks, but there's no
particular guarantee that that's not going to be a symlink too. Using the
canonicalized temp dir by default is also slightly nicer for a couple reasons:
It's sometimes faster (an in-memory tempfs on e.g. Arch), and it makes
overriding `$TMPDIR` or `%TMP%` sufficient to control where tests put temp
files, without needing to override `UV_INTERNAL__TEST_DIR` too.

.github/workflows/setup-dev-drive.ps1

@@ -85,7 +85,6 @@ Write-Output `
 	"DEV_DRIVE=$($Drive)" `
 	"TMP=$($Tmp)" `
 	"TEMP=$($Tmp)" `
-	"UV_INTERNAL__TEST_DIR=$($Tmp)" `
 	"RUSTUP_HOME=$($Drive)/.rustup" `
 	"CARGO_HOME=$($Drive)/.cargo" `
 	"UV_WORKSPACE=$($Drive)/uv" `