add kepr.build — enables Kepr Builds for all four platforms on every offer

@@ -1,7 +1,14 @@11 const std = @import("std");22 33 pub fn build(b: *std.Build) void {4-    const target = b.standardTargetOptions(.{});4+    // When built by Kepr Builds, KEPR_TARGET carries the cross-compile triple5+    // so the same command works across all platforms without shell expansion.6+    const target = if (std.c.getenv("KEPR_TARGET")) |t_ptr|7+        b.resolveTargetQuery(std.Target.Query.parse(8+            .{ .arch_os_abi = std.mem.span(t_ptr) },9+        ) catch std.Target.Query{})10+    else11+        b.standardTargetOptions(.{});512     const optimize = b.standardOptimizeOption(.{});613 714     const exe = b.addExecutable(.{