The metal detectors are a recent and unnecessary addition to the capitol building. The Governor was against their addition as well. Dan Patrick was the main person who pushed for them.
Normally they cannot restrict access under 30.06
(e) It is an exception to the application of this section that the property
on which the license holder carries a handgun is owned or leased
by a governmental entity and is not a premises or other place on which
the license holder is prohibited from carrying the handgun under
Section 46.03 or 46.035.
30.06 signs may be posted though while the TX congress is in session. See 46.035.