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/nix-daemon/nix-daemon.cc

@@ -1051,7 +1051,6 @@ static void daemonLoop(char** argv) {
       options.errorPrefix = "unexpected Nix daemon error: ";
       options.dieWithParent = false;
       options.runExitHandlers = true;
-      options.allowVfork = false;
       startProcess(
           [&]() {
             fdSocket = -1;