Change INSTALL_DIR to OTEL_DOTNET_AUTO_HOME in scripts

[pellared]

Why

Part of #1373

1. User can define a global machine-wide (or user-wide) OTEL_DOTNET_AUTO_HOME so that INSTALL_DIR (which is not a unique name) does not have to be provided when a custom installation path is used
   1. At some point install.sh could even set OTEL_DOTNET_AUTO_HOME as a global env var to facilitate using ./instrument.sh. Not now for sure - personally I want to postpone setting global env vars until we see that it is beneficial)
2. Takes advantage of existing env var name which is required anyway

Also, change the default so that, by default, the ./instrument.sh could be used from a different location.

What

• Change INSTALL_DIR env var script param to OTEL_DOTNET_AUTO_HOME
• Change the default installation path to $HOME/.otel-auto-dotnet
  • Based on what dotnet-install.sh
  • It is not dependent on $PWD
  • Does not require admin rights
• Remove RELEASES_URL script param (YAGNI)

Tests

CI

Checklist

• CHANGELOG.md is updated.
• Documentation is updated.
• New features are covered by tests.

[rajkumar-rangaraj]

LGTM