Expose Illumos commit used in builds in helios.json

[wfchandler]

Currently the helios.json file generated during TUF repo builds does not list the illumos-gate commit it used.

This can be found by searching for os_short_commit in the build log as below, but it would be more convenient if we showed it more directly.

[8511](https://buildomat.eng.oxide.computer/wg/0/details/01JVJTA28DEMM4C4ARXQ5W5WWX/3iCCLG9ZSw1N5rB3A5vMPbhfyl2plK7Ey3nKO8G0dR53pl14/01JVJTABRHWDD246MWWB6HACXJ#S8511)	2025-05-18T23:52:40.087Z	[  recovery-image] May 18 23:52:39.356 INFO expanded image name: "recovery dbb6669/${os_short_commit} 2025-05-18 23:17" -> "recovery dbb6669/361574b 2025-05-18 23:17"