Fix std.heap.pageSize() not being comptime-known

[QSmally]

With some targets, e.g. tested with aarch64-linux-musl, std.heap.pageSize() is not comptime-known in compiler version 0.16.0 and therefore returns an error:

zig-pkg/zmpl-0.0.1-SYFGBq-yAwCLE5V8niLARi8eny-pM3FuzEQ18yhcX7jo/src/zmpl/debug.zig:125:32: note: called at comptime from here
    var buf: [std.heap.pageSize()]u8 = undefined;
              ~~~~~~~~~~~~~~~~~^~
zig-pkg/zmpl-0.0.1-SYFGBq-yAwCLE5V8niLARi8eny-pM3FuzEQ18yhcX7jo/src/zmpl/debug.zig:125:32: note: types must be comptime-known

This patch uses std.heap.page_size_min which is always comptime-known (which std.heap.pageSize() conditionally returns at comptime).