use general purpose allocator for string concatenation

gpa.zig

const std = @import("std");
var gpa = std.heap.GeneralPurposeAllocator(.{}){};
const alloc = gpa.allocator();

pub fn main() !void {
    var distance: i32 = 7857;
    var str = std.fmt.allocPrint(
        alloc,
        "test alloc: {d}",
        .{distance},
    ) catch "format failed";
    defer alloc.free(str);

    std.debug.print("{s}", .{str});
}