Extension installer needs to listen to the APPSIGNAL_CA_FILE_PATH env var

[tombruijn]

The AppSignal Elixir integration needs to listen to the APPSIGNAL_CA_FILE_PATH env var in the extension installer.

To download the archive, some additional options may be set.

• SSL. Every integration includes a CA certificates file. This is used to configure SSL for the request to download the archive. This is not configurable with the APPSIGNAL_CA_FILE_PATH environment variable. It's always set to use the included CA certificates file.

From https://github.com/appsignal/integration-guide/blob/7cf3b14f89c6cf8d84048ae54a5e739974410b02/build/installation.md#download-options

To do

• Listen to APPSIGNAL_CA_FILE_PATH env var in mix helpers file.
• Add tests for the cacert behavior in the mix helpers test file.