Require every shell for the parity tests in the Linux unit job

The Linux unit test job installs zsh, so it now sets `FX_SHELL_PARITY_ALL_SHELLS` and the shell parity tests fail there instead of skipping when sh, dash, bash, or zsh is missing.

.github/workflows/ci.yml

@@ -64,7 +64,9 @@ jobs:
       # The zsh user profile tests in src/core/execution/command_runner.zig
       # skip without /bin/zsh. They start interactive zsh with no terminal,
       # where Ubuntu's global zshrc runs compinit, and on this runner compinit
-      # aborts and writes an error to stderr, so the job turns it off.
+      # aborts and writes an error to stderr, so the job turns it off. The
+      # shell parity tests in src/core/shell_command/command_effect.zig use
+      # zsh too.
       - name: Install zsh
         run: |
           command -v zsh >/dev/null || { sudo apt-get update -qq && sudo apt-get install -y -qq zsh; }
@@ -73,6 +75,9 @@ jobs:
       - name: Test
         env:
           TEST_TIMEOUT_SECONDS: ${{ matrix.test_timeout_seconds }}
+          # Ubuntu has sh, dash, and bash, and the step above adds zsh, so the
+          # shell parity tests fail here if any of them is missing.
+          FX_SHELL_PARITY_ALL_SHELLS: "1"
         run: |
           started_at=$SECONDS
           trap 'printf "Unit tests elapsed: %ss\n" "$((SECONDS - started_at))"' EXIT

src/core/shell_command/command_effect.zig

@@ -2073,9 +2073,10 @@ fn checkShellParity(command: []const u8, mode: ParityMode) !void {
     }
 }
 
-/// Finds the shell, or returns null to skip it. The nightly Fuzz workflow
-/// installs every shell and sets `FX_SHELL_PARITY_ALL_SHELLS`, so a missing
-/// shell there fails instead of quietly narrowing the check.
+/// 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.
 fn parityShellPath(shell: ParityShell) !?[]const u8 {
     for (shell.paths) |path| {
         std.Io.Dir.accessAbsolute(std.testing.io, path, .{}) catch continue;