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/libstore/build.cc

@@ -2342,8 +2342,6 @@ void DerivationGoal::startBuilder() {
 
     userNamespaceSync.create();
 
-    options.allowVfork = false;
-
     Pid helper = startProcess(
         [&]() {
           /* Drop additional groups here because we can't do it
@@ -2443,7 +2441,6 @@ void DerivationGoal::startBuilder() {
 #endif
   {
   fallback:
-    options.allowVfork = !buildUser && !drv->isBuiltin();
     pid = startProcess([&]() { runChild(); }, options);
   }