Refactor tegola_lambda CI to use Amazon Linux for building

[ARolek]

AWS Lambda uses Amazon Linux for Lambda executions. The previous build pipeline was using Ubuntu 18.04 to build tegola_lambda which had GLIB location parity with Amazon Linux. After upgrading to Ubuntu 20.04 the GLIB references between Ubuntu and Amazon Linux are no longer in parity so tegola_lambda fails to find GLIB during Lambda execution.

This commit accomplished the following:

• build tegola_lambda using Amazon Linux rather than Ubuntu
• move from using ubuntu-latest to ubuntu-20.04 in GH actions.

closes #789

[coveralls]

Pull Request Test Coverage Report for Build 7564919ab-PR-790

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 45.052%

---

Totals
Change from base Build 126d51113:	0.0%
Covered Lines:	5440
Relevant Lines:	12075

---

💛 - Coveralls