Read the shell parity setting from the test environment

Tests never set up the environment `io.getenv` reads, so the shell parity tests never saw `FX_SHELL_PARITY_ALL_SHELLS` and skipped a missing shell even when it was set. They now read it from `std.testing.environ`.

src/core/shell_command/command_effect.zig

@@ -2080,13 +2080,14 @@ fn checkShellParity(command: []const u8, mode: ParityMode) !void {
 /// Finds the shell, or returns null to skip it. The Linux unit test job and
 /// the nightly Fuzz workflow install every shell and set
 /// `FX_SHELL_PARITY_ALL_SHELLS`, so a missing shell there fails instead of
-/// quietly narrowing the check.
+/// quietly narrowing the check. Tests read the process environment from
+/// `std.testing.environ`, since fx's own environment is never set up in them.
 fn parityShellPath(shell: ParityShell) !?[]const u8 {
     for (shell.paths) |path| {
         std.Io.Dir.accessAbsolute(std.testing.io, path, .{}) catch continue;
         return path;
     }
-    if (@import("../shared/io.zig").getenv("FX_SHELL_PARITY_ALL_SHELLS") != null) {
+    if (try std.testing.environ.contains(std.testing.allocator, "FX_SHELL_PARITY_ALL_SHELLS")) {
         // The failure output names the path where the shell was expected.
         std.testing.expectEqualStrings(shell.paths[0], "") catch {};
         return error.TestShellParityShellMissing;