build: pass zig exe path to combine_archives

`combine_archives` spawns `zig ar -M` to combine static archives via
an MRI script. It hard-coded the command name `"zig"` and relied on
the binary being on `PATH`, which fails on Windows when the build is
driven by an absolute zig.exe path (common in CI and in Scoop/winget
installs where PATH isn't populated at build time). The failure
surfaces as `error: FileNotFound` from `Child.spawn`.

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

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;