Dynamic allocation for file descriptors used in ./build-host.sh

Visual studio code had an issue with file descriptors already used by the integrated terminal.
This commit asks bash to dynamically assign a "free" file descriptor to be used in the expect function.
Closes #135
6 jobs for fd200 in 28 minutes and 24 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Lint
passed shellcheck #15663

00:00:13

passed shfmt #15664

00:00:14

 
  Build
passed build #15665
fast-single-thread

00:24:08

 
  Test
passed test-basic-qemu #15667
fast-single-thread

00:03:06

passed test-cloudimg-qemu #15668
fast-single-thread

00:03:58

passed test-vagrant-boxes-format #15666

00:00:42