Allow setting apppack.toml location in environment variable

[ipmb]

Some applications may want to run different services in different environments. For example a production application might run specific background workers for priority tasks while non-production runs all background workers as a single service.

If the user sets an APPPACK_TOML environment variable, the builder should try to load TOML from that path. If it doesn't exist, the build should fail.

An example would be a user setting apppack -a <app_name> config set APPPACK_TOML=apppack.minimal.toml. Subsequent builds should load apppack.minimal.toml instead of apppack.toml.

This will require updating some hard-coded references. Here are a couple examples, but there may be others:

• apppack-codebuild-image/builder/build/prebuild.go

  Line 151 in 9a75db4

  apppackTomlExists, err := b.state.FileExists("apppack.toml")

• apppack-codebuild-image/builder/build/apppacktoml.go

  Line 106 in 9a75db4

  if _, err := os.Stat("apppack.toml"); os.IsNotExist(err) {

[ipmb]

Closed in #11