Related to https://github.com/zed-industries/zed/pull/44109,
https://github.com/zed-industries/zed/issues/43884,
https://github.com/zed-industries/zed/issues/43809.

In the Linux dispatcher, we create one background thread per CPU, but
when a single core is available, having a single background thread
significantly hinders the perceived performance of Zed. This is
particularly helpful when SSH remoting to low-resource servers.

We may want to bump this to more than two threads actually, but I wanted
to be conservative, and this seems to make a big difference already.

Release Notes:

- N/A