feat: unique directory for pipelines

Also, start using fork/chdir/exec idiom instead of posix_spawn, because
as we all know: posix_spawn is stupid as a system call:

https://lwn.net/Articles/360556/

include/cli.h

@@ -29,6 +29,7 @@ typedef struct {
     bool use_colors;
     optional_str log_file;
     optional_str pipeline_log_dir;
+    optional_str pipeline_cwd;
     optional_strlist environment_vars;
 } cli_options;