feat: 3rd use environment vars to workaround issues

[noelmcloughlin]

This PR does the following:

• improved variable names and better documentation in config.env and makefile

• introduce LINKML_ENVIRONMENT_FILE var for custom configuration location. For example to hide the file via .config.yaml or .config/generators.yaml or some other odd reason.

• Add some logic to Makefile to use environment variables as action triggers

1. set typescript environment variable (args) to trigger gen-typescript
2. set java environment variable (args) to trigger gen-java
3. set owl environment variable (args) if rdfs is needed (Parm 'owl: {metadata_profile: rdfs} in config.yaml error linkml#1453)

This PR replaces #69

[noelmcloughlin]

See also #95 and comments in #57