Allow to configure repository license using environment variable

[jsoriano]

Allow to use ELASTIC_PACKAGE_REPOSITORY_LICENSE to select the default repository license to use in a repository with multiple licenses. Previously the file LICENSE.txt at the root of the repository was used.

Required for elastic/integrations#4593.

[mrodm]

👍

[elasticmachine]

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• Start Time: 2022-11-08T16:58:43.726+0000

• Duration: 30 min 26 sec

Test stats 🧪

Test	Results
Failed	0
Passed	863
Skipped	0
Total	863

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🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (35/35)	💚
Files	66.667% (86/129)	👍
Classes	61.413% (113/184)	👍
Methods	47.662% (367/770)	👍
Lines	30.723% (3288/10702)	👍
Conditionals	100.0% (0/0)	💚