build: pass zig exe path to combine_archives (#12382)

`combine_archives` spawns `zig ar -M`, hard-coding the command name
`"zig"` and relying on the binary being on `PATH`. On Windows when the
build is driven by an absolute zig.exe path (common in CI and
Scoop/winget installs), this surfaces as `error: FileNotFound`.

Pass `b.graph.zig_exe` explicitly so the tool always uses the exact zig
binary driving the build, matching how other build tools in this repo
spawn zig subcommands.

Part of the Win32 apprt upstreaming series (see discussion #2563 /
mattn/ghostty#1).

src/build/GhosttyLibVt.zig

@@ -339,6 +339,7 @@ fn combineArchives(
         }),
     });
     const run = b.addRunArtifact(tool);
+    run.addArg(b.graph.zig_exe);
     const output = run.addOutputFileArg("libghostty-vt.a");
     for (sources) |source| run.addFileArg(source);
 

src/build/combine_archives.zig

@@ -7,7 +7,7 @@
 //! available on Windows. This tool handles both the script generation
 //! and the piping in a single cross-platform executable.
 //!
-//! Usage: combine_archives <output.a> <input1.a> [input2.a ...]
+//! Usage: combine_archives <zig_exe> <output.a> <input1.a> [input2.a ...]
 
 const std = @import("std");
 
@@ -16,13 +16,14 @@ pub fn main() !void {
     const alloc = gpa.allocator();
 
     const args = try std.process.argsAlloc(alloc);
-    if (args.len < 3) {
-        std.log.err("usage: combine_archives <output> <input...>", .{});
+    if (args.len < 4) {
+        std.log.err("usage: combine_archives <zig_exe> <output> <input...>", .{});
         std.process.exit(1);
     }
 
-    const output_path = args[1];
-    const inputs = args[2..];
+    const zig_exe = args[1];
+    const output_path = args[2];
+    const inputs = args[3..];
 
     // Build the MRI script.
     var script: std.ArrayListUnmanaged(u8) = .empty;
@@ -36,7 +37,7 @@ pub fn main() !void {
     }
     try script.appendSlice(alloc, "SAVE\nEND\n");
 
-    var child: std.process.Child = .init(&.{ "zig", "ar", "-M" }, alloc);
+    var child: std.process.Child = .init(&.{ zig_exe, "ar", "-M" }, alloc);
     child.stdin_behavior = .Pipe;
     child.stdout_behavior = .Inherit;
     child.stderr_behavior = .Inherit;