Don't fail when sigstore logs to stderr

[hugovk]

Sigstore v3 used to print the "Transparency log entry created at index:" lines, but v4 logs them to stderr instead:

• https://github.com/sigstore/sigstore-python/blob/v3.6.7/sigstore/_cli.py#L706-L708
• https://github.com/sigstore/sigstore-python/blob/v4.0.0/sigstore/_cli.py#L683-L685
• Parallel signing sigstore/sigstore-python#1468

When we run sigstore, we treat anything on stderr as an error, and it now falls over.

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Traceback (most recent call last):
File "/Users/hugo/github/release-tools/run_release.py", line 1508, in <module>
main()
~~~~^^
File "/Users/hugo/github/release-tools/run_release.py", line 1504, in main
automata.run()
~~~~~~~~~~~~^^
File "/Users/hugo/github/release-tools/run_release.py", line 265, in runraise e fromNone
File "/Users/hugo/github/release-tools/run_release.py", line 262, in runself.current_task(self.db)
~~~~~~~~~~~~~~~~~^^^^^^^^^
File "/Users/hugo/github/release-tools/release.py", line 153, in __call__returngetattr(self, "function")(db)
~~~~~~~~~~~~~~~~~~~~~~~~~^^^^
File "/Users/hugo/github/release-tools/run_release.py", line 1104, in run_add_to_python_dot_orgraise paramiko.SSHException(f"Failed to execute the command: {stderr_text}")
paramiko.ssh_exception.SSHException: Failed to execute the command: [14:11:19] INFO Transparency log entry created at index: _cli.py:7292340619340
[14:11:21] INFO Transparency log entry created at index: _cli.py:729
2340619402
[14:11:22] INFO Transparency log entry created at index: _cli.py:729
2340619453
[14:11:24] INFO Transparency log entry created at index: _cli.py:729
2340619530
[14:11:26] INFO Transparency log entry created at index: _cli.py:729
2340619584
[14:11:28] INFO Transparency log entry created at index: _cli.py:729
2340619639
[14:11:29] INFO Transparency log entry created at index: _cli.py:729
2340619685
[14:11:31] INFO Transparency log entry created at index: _cli.py:729
2340619733
[14:11:33] INFO Transparency log entry created at index: _cli.py:729
2340619793
[14:11:35] INFO Transparency log entry created at index: _cli.py:729
2340619861
[14:11:37] INFO Transparency log entry created at index: _cli.py:729
2340619926
[14:11:38] INFO Transparency log entry created at index: _cli.py:729
2340619988
[14:11:40] INFO Transparency log entry created at index: _cli.py:729
2340620035

Instead, let's print the stderr, and only fail on non-zero output.