Prior to this fix, the compare-outputs test suite was showing a strange
behavior, the tests always stopped between tests 6-8 and had a stack track
similar to each other.
```
Test 8/68 compare-output multiple files with private function (ReleaseSmall)...OK
/usr/home/mgxm/dev/zig/zig-cache/source.zig:7:2: error: invalid token: '&'
}&(getStdOut() catch unreachable).outStream().stream;
^
The following command exited with error code 1:
```
With the wrong O_* flags, the source code was being written in append mode which
resulted in an invalid file
```zig
use @import("foo.zig");
use @import("bar.zig");
pub fn main() void {
foo_function();
bar_function();
}&(getStdOut() catch unreachable).outStream().stream;
stdout.print("OK 2\n") catch unreachable;
}
fn privateFunction() void {
printText();
}
```
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