Avoid zsh taking over /dev/tty in self-test (#1557)

After running nix-installer self-test in a script on Darwin, if I then
attempt to read from /dev/tty in the same script, it gets suspended by
SIGTTIN. This seems to be a result of running zsh -ic which appears to
take over /dev/tty via tcsetpgrp when doing job control stuff[1]. I can
replicate the issue by running sh -c 'zsh -ic /usr/bin/true && read'. I
haven't traced the exact codepaths, but the job control stuff only seems
to kick in when zsh spawns and waits on a subprocess, so by execing the
command, we seem to avoid the problem. This also avoids keeping the
shell around longer than necessary so this seems like a win-win.

[1] https://github.com/zsh-users/zsh/blob/87c055fb4d076c0d9bfffbef16ac8dd9348903df/Src/utils.c#L4779-L4781

Author: joshheinrichs-shopify

src/self_test.rs

@@ -105,7 +105,7 @@ impl Shell {
             .as_millis();
 
         command.arg(format!(
-            r#"nix build --option substitute false --option post-build-hook '' --no-link --expr 'derivation {{ name = "self-test-{executable}-{timestamp_millis}"; system = "{SYSTEM}"; builder = "/bin/sh"; args = ["-c" "echo hello > \$out"]; }}'"#
+            r#"exec nix build --option substitute false --option post-build-hook '' --no-link --expr 'derivation {{ name = "self-test-{executable}-{timestamp_millis}"; system = "{SYSTEM}"; builder = "/bin/sh"; args = ["-c" "echo hello > \$out"]; }}'"#
         ));
         let command_str = format!("{:?}", command.as_std());