build: set overall timeout to 72 hours

Some builds are pending for weeks now because of the global time limit. Give them a chance to be built.

I've made a small research on the data since 1 January 2021. Idle time estimation assumes that compilation took 36 hours and deployment took 20 minutes.

Date of deployment	Hours since previous deployment	Hours idling
2021-03-06 21:22	60.32	23.99
2021-03-04 09:03	75.33	39
2021-03-01 05:43	55.33	19
2021-02-26 22:23	57.45	21.12
2021-02-24 12:56	54.08	17.75
2021-02-22 06:51	46.88	10.55
2021-02-20 07:58	49.4	13.07
2021-02-18 06:34	70.38	34.05
2021-02-15 08:11	72.9	36.57
2021-02-12 07:17	214.67	178.34
2021-02-03 08:37	48.48	12.15
2021-02-01 08:08	67.52	31.19
2021-01-29 12:37	50.77	14.44
2021-01-27 09:51	95.33	59
2021-01-23 10:31	51.22	14.89
2021-01-21 07:18	154.65	118.32
2021-01-14 20:39	54.82	18.49
2021-01-12 13:50	54.42	18.09
2021-01-10 07:25	57.77	21.44
2021-01-07 21:39	52.38	16.05
2021-01-05 17:16	50.77	14.44
2021-01-03 14:30	41.67	5.34
2021-01-01 20:50

Summary: the build server was idling 737 hours out of 1537. That's 48%!

The calculations don't take into consideration publish/gpgsign, update, signindex which can take a lot of time.