Inherit os.Stdin directly from the parent process

[myaaaaaaaaa]

This fixes a bug where termshot command </dev/null (common in non-interactive environments such as CI) can result in the output of command being truncated.

This bug is due to a race condition where the instant EOF of /dev/null can cause defer func() { _ = pt.Close() }() to run before the output of command is finished writing. Since stdin, stdout, and stderr all refer to the same pty, this also closes stdout, which results in the rest of command's output being discarded.

Partially fixes #412

[myaaaaaaaaa]

It's worth noting that the original version didn't even work in the first place - since both the child process and termshot itself were simultaneously reading from the same pseudo-terminal device, termshot can end up consuming the data meant for the child process. For example:

# Want
$ echo hi | sleep 0

# Got (before this PR is applied)
$ echo hi | termshot sleep 0
hi

# Got (after this PR is applied)
$ echo hi | termshot sleep 0