Some local env variables can't be set using netlify.toml

[culturestate]

Describe the bug

For local plugin development, I need to access some Netlify environment variables that aren't provided by build - SITE_ID for API calls and DEPLOY_ID and BUILD_ID to generate a mock UI. Using dotenv to set these manually works as expected, but when I set them using netlify.toml, only SITE_ID is passed. This is semi-related to #1514.

I know these are described as read-only in the documentation, but:

• I can set SITE_ID, so it seems like I should be able to set them all
• It would be helpful if local environments could override these vars

Configuration

You can see the calls at https://github.com/quarva/netlify-plugin-sitrep/blob/master/index.js - builds are run using ntl build and an untracked local netlify.toml, configured as below.

envinfo

  System:
    OS: macOS 10.15.6
    CPU: (8) x64 Intel(R) Core(TM) i7-8559U CPU @ 2.70GHz
    Memory: 339.12 MB / 16.00 GB
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 14.9.0 - ~/.nvm/versions/node/v14.9.0/bin/node
    Yarn: 1.22.5 - /usr/local/bin/yarn
    npm: 6.14.8 - ~/.nvm/versions/node/v14.9.0/bin/npm
  npmPackages:
    @netlify/build: ^3.3.5 => 3.3.5 
  npmGlobalPackages:
    netlify-cli: 2.61.2

Logs
dotenv

.env

SITE_ID="SiteIDFromDotEnv"
BUILD_ID=BuildIDFromDotEnv
DEPLOY_ID=DeployIDFromDotEnv`

console.log(process.env)

 { ...
 SITE_ID: 'SiteIDFromDotEnv',
 BUILD_ID: 'BuildIDFromDotEnv',
 DEPLOY_ID: 'DeployIDFromDotEnv'
}

netlify.toml

netlify.toml

[build.environment]
SITE_ID="SiteIDFromNetlify.toml"
BUILD_ID="BuildIDFromNetlify.toml"
DEPLOY_ID="DeployIDFromNetlify.toml"

console.log(process.env)

{ ...
SITE_ID: 'SiteIDFromNetlify.toml',
... }

And for a sanity check:

console.log(SITE_ID + '-' + BUILD_ID + '-' + DEPLOY_ID);

SiteIDFromNetlify.toml-undefined-undefined

[culturestate]

After looking at https://github.com/netlify/build/blob/master/packages/build/src/env/main.js I can see that this is expected behavior; the ability to set SITE_ID even though it's marked in the docs as a read-only variable threw me off.

With that in mind, I'll close this issue and hope for .env support in the future :)