Avoid spawning commands

[SergioGasquez]

• Avoid using hardcoded len for the mscp
• For some reason, when using spawn, some CIs were failing . I think what was happening was that the await was completed but the command was still running which lead to some parts of the toolchain not being properly installed.
  • This slows down the installation a bit
  • The same CI with this changes now works fine https://github.com/SergioGasquez/esp-flasher-stub/actions/runs/3902025238

I was able to reproduce this issue in the CI and using doing the installation in a Dockerfile but not on host machines and neither on esp-hal CI.

I plan to create a new espup release once this is fixed, so we can update the rest of the CIs

[jessebraham]

LGTM