fix(3p/nix): do not call vfork

The use of vfork() in Nix is entirely illegal. Quote:

If the process created by vfork() returns from the function in which vfork() was
called, or calls any other function before successfully calling _exit() or
one of the exec*() family of functions, the behavior is undefined.

-- Linux man-pages, release 5.05

Add a TODO to use the higher-performance variants of clone() on Linux when it
is available.

Change-Id: I42370e1568ad6e2d00d70d0b66c8aded8f1288bb
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1418
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>
Reviewed-by: Alyssa Ross <hi@alyssa.is>

third_party/nix/src/libutil/util.hh

@@ -232,7 +232,6 @@ struct ProcessOptions {
   std::string errorPrefix = "error: ";
   bool dieWithParent = true;
   bool runExitHandlers = false;
-  bool allowVfork = true;
 };
 
 pid_t startProcess(std::function<void()> fun,