[VL] Skip the velox download if velox_branch not exists

[FelixYBW]

When velox_home is specified and velox_repo:velox_branch doesn't exists, it means user prepared the customized Velox. We can skip the velox download and continue the build. It's user's responsibility to make sure the velox has correct version.

The PR served 2 purpose:

1. User can skip the automatic Velox download if they prepared customized velox and use a fake velox_branch
2. User can't access the velox_repo for some reason and would like to prepare the velox source code by themselves

[github-actions]

Thanks for opening a pull request!

Could you open an issue for this pull request on Github Issues?

https://github.com/apache/incubator-gluten/issues

Then could you also rename commit message and pull request title in the following format?

[GLUTEN-${ISSUES_ID}][COMPONENT]feat/fix: ${detailed message}

See also:

• Other pull requests

[zhouyuan]

+1
works for me

[GlutenPerfBot]

===== Performance report for TPCH SF2000 with Velox backend, for reference only ====

query	log/native_master_02_11_2025_time.csv	log/native_master_02_10_2025_39950cf26a_time.csv	difference	percentage
q1	25.47	25.78	0.305	101.20%
q2	26.31	27.04	0.732	102.78%
q3	32.86	33.01	0.152	100.46%
q4	27.43	27.69	0.259	100.94%
q5	60.53	59.77	-0.756	98.75%
q6	9.00	9.22	0.219	102.44%
q7	38.84	40.14	1.300	103.35%
q8	64.26	62.20	-2.054	96.80%
q9	97.16	100.16	3.002	103.09%
q10	43.94	44.16	0.215	100.49%
q11	18.83	16.76	-2.070	89.01%
q12	15.97	16.67	0.699	104.37%
q13	25.04	27.33	2.289	109.14%
q14	12.93	13.23	0.301	102.33%
q15	26.64	28.64	1.999	107.51%
q16	12.78	13.61	0.824	106.44%
q17	78.45	78.55	0.095	100.12%
q18	119.56	119.53	-0.023	99.98%
q19	13.44	16.14	2.698	120.08%
q20	25.77	26.01	0.242	100.94%
q21	187.32	185.95	-1.369	99.27%
q22	10.47	13.05	2.585	124.70%
total	973.00	984.64	11.644	101.20%