Fix versionagedays() misparsing build time as a date

[vairakkumaarsvs]

versionagedays() split the version string on "_" and always took index 1 as the YYYYMMDD build date. That index is only correct for the Buck build path (gen_version.py), which prepends a MAIN_VERSION component so the string is "x.y.z_YYYYMMDD_HHMMSS_hash" (date at index 1). The open-source build.py path's auto_version() omits that prefix, producing "YYYYMMDD_HHMMSS_hash" instead - so index 1 is actually the HHMMSS time portion, not the date.

Because strptime's "%Y" greedily consumes exactly 4 digits, a 6-digit time string like "044315" doesn't fail to parse - it silently succeeds as year 0443, and the resulting multi-century "old version" hint gets shown even right after a fresh build.

Scan for the first 8-digit, all-numeric "_"-delimited part instead of assuming a fixed index, so this works for both version string shapes.

[meta-codesync]

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