[shell.nix] add bun install --dev for spago@next etc

3 jobs for dev-spago-next in 5 minutes and 20 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Deps
passed #12897
deps

00:02:33

 
  Compile
failed #12898
compile

00:02:46

 
  Test
skipped #12899
test
 
Name Stage Failure
failed
compile Compile
bun install v1.0.13 (f5bf67bd)

Checked 1087 installs across 1039 packages (no changes) [12.00ms]
Compiling
$ spago build
/nix/store/gabs49v4yacq6hdd1s5bnjyfrx1yaan3-bash-interactive-5.2-p15/bin/bash: line 1: spago: command not found
error: script "build" exited with code 127
Cleaning up file based variables
ERROR: Job failed: exit code 1