nix: develop: always force SHELL to chosen shell

SHELL was inherited from the system environment. This resulted in a new
shell being started, but with SHELL still referring to the system shell
and not the one used by nix-develop.

Applications like make, use SHELL to run commands, which meant that
top-level commands are run inside the nix-develop-shell, but
sub-commands are ran inside the system shell.

This setenv forces SHELL to always be set to the shell used by
nix-develop.

src/nix/develop.cc

@@ -293,7 +293,6 @@ struct Common : InstallableCommand, MixProfile
         "NIX_LOG_FD",
         "NIX_REMOTE",
         "PPID",
-        "SHELL",
         "SHELLOPTS",
         "SSL_CERT_FILE", // FIXME: only want to ignore /no-cert-file.crt
         "TEMP",
@@ -643,6 +642,10 @@ struct CmdDevelop : Common, MixEnvironment
             ignoreException();
         }
 
+        // Override SHELL with the one chosen for this environment.
+        // This is to make sure the system shell doesn't leak into the build environment.
+        setenv("SHELL", shell.data(), 1);
+
         // If running a phase or single command, don't want an interactive shell running after
         // Ctrl-C, so don't pass --rcfile
         auto args = phase || !command.empty() ? Strings{std::string(baseNameOf(shell)), rcFilePath}