feat(cli): re-exec with DYLD fallback paths set

The engine's dlopen of libkrunfw and mke2fs's of libarchive use bare
sonames, and dyld reads DYLD_FALLBACK_LIBRARY_PATH once, at process
startup — a setenv afterwards is invisible to it. When the variable
is unset and the brew kegs exist, the CLI sets it and execv's itself:
exec restarts dyld, which reads the variable fresh. Library consumers
set the variable before their process starts instead (SPEC.md).

Cargo.lock

@@ -385,6 +385,7 @@ dependencies = [
  "clap",
  "ecrou",
  "indicatif",
+ "libc",
  "log",
 ]