fix: isolate updater service test files

[immanuwell]

What

Updater service tests used filepath.Join(os.TempDir(), "test") as a shared fake binary path. If /tmp/test already exists as a dir, the package falls over before it tests updater logic. tiny papercut, but pretty easy to hit on a dev box.

This swaps it to t.TempDir() so each test gets its own scratch file. No prod code touched.

Repro

mkdir -p /tmp/test
go test ./cmd/cloudflared/updater

Before: open /tmp/test: is a directory

After: updater tests pass.

Tested

go test ./cmd/cloudflared/updater
go test -race -mod=vendor ./cmd/cloudflared/updater
go test ./...

go test ./... still hits local ingress ICMP permission failures here, unrelated to this change.