CMake-style ConfigHeader deletes last line of input file

[distractedlambda]

Zig Version

0.11.0-dev.3803+7ad104227

Steps to Reproduce and Observed Behavior

Create a project with a build.zig and config.h.in:

• build.zig:

const std = @import("std");

pub fn build(b: *std.Build) void {
    const cmake_config = b.addConfigHeader(.{ .style = .{ .cmake = .{ .path = "config.h.in" } } }, .{});
    const install_cmake_config = b.addInstallFile(cmake_config.getFileSource(), "config-cmake.h");
    b.getInstallStep().dependOn(&install_cmake_config.step);

    const autoconf_confing = b.addConfigHeader(.{ .style = .{ .autoconf = .{ .path = "config.h.in" } } }, .{});
    const install_autoconf_config = b.addInstallFile(autoconf_confing.getFileSource(), "config-autoconf.h");
    b.getInstallStep().dependOn(&install_autoconf_config.step);
}

• config.h.in:

#define FOO
#define BAR

Then run zig build, yielding these outputs:

• zig-out/config-cmake.h:

/* This file was generated by ConfigHeader using the Zig Build System. */
#define FOO

• zig-out/config-autoconf.h:

/* This file was generated by ConfigHeader using the Zig Build System. */
#define FOO
#define BAR

Expected Behavior

I would expect both generated config files to have the same effective contents.

[Jan200101]

introduced by #16037, simple fix will be PR'd soon