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).

crates/ecrou-cli/Cargo.toml

@@ -12,6 +12,7 @@ name = "ecrou"
 path = "src/main.rs"
 
 [dependencies]
+libc = "0.2"
 ecrou = { path = "../ecrou" }
 
 # CLI parsing