[android] Skip source fetch during android build-docker

[justice-adams-apple]

• Add option to skip-fetch so CI users can call fetch-source and build-docker independently as needed
• Pass necessary env variables for CI environment
• Fix pearl substitution flags

This should allow us to rework our CI job for PR testing to use build-docker

[finagolfin]

If this is working in your testing, looks good to me.

[finagolfin]

@shahmishal, can @justice-adams-apple or somebody else you trust be made a code owner on this Android SDK directory alone in this repo as part of this pull, so we can make Android changes more quickly? I want to whittle these Android scripts down to just downloading the NDK, running a build-script preset, and finally running swift-sdk-generator on the resulting cross-compilation toolchain, greatly simplifying this Docker setup run on the Android CI.

It would be better if I could do that as a series of smaller pulls that Justice or somebody else can review in the coming months, rather than waiting for you to review all that, as I'm sure you're very busy. Once all that is in, all future config would be done in those toolchain repos instead, and changes here would be minimal.

[shahmishal]

@shahmishal, can @justice-adams-apple or somebody else you trust be made a code owner on this Android SDK directory alone in this repo as part of this pull, so we can make Android changes more quickly?

Yes, #539

[finagolfin]

Thanks, Justice, I hope I didn't sign you up for a bunch more CI work you don't want, 😉 though at least with my Android pulls, I will ask other Android workgroup members like @marcprux, who put together a significant portion of these CI scripts with me, to review also.

[finagolfin]

@justice-adams-apple, more Android CI work required to get this in? I'd like to rebase #536 on this and get that in next.