feat(core): enable plugin isolation by default (#26888)

<!-- Please make sure you have read the submission guidelines before
posting an PR -->
<!--
https://github.com/nrwl/nx/blob/master/CONTRIBUTING.md#-submitting-a-pr
-->

<!-- Please make sure that your commit message follows our format -->
<!-- Example: `fix(nx): must begin with lowercase` -->

<!-- If this is a particularly complex change or feature addition, you
can request a dedicated Nx release for this pull request branch. Mention
someone from the Nx team or the `@nrwl/nx-pipelines-reviewers` and they
will confirm if the PR warrants its own release for testing purposes,
and generate it for you if appropriate. -->

## Current Behavior
<!-- This is the behavior we have today -->

## Expected Behavior
<!-- This is the behavior we should expect with the changes in this PR
-->

## Related Issue(s)
<!-- Please link the issue being fixed so it gets closed when this is
merged. -->

Fixes #

packages/nx/src/project-graph/plugins/internal-api.ts

@@ -151,7 +151,7 @@ export async function loadNxPlugins(
   performance.mark('loadNxPlugins:start');
 
   const loadingMethod =
-    process.env.NX_ISOLATE_PLUGINS === 'true'
+    process.env.NX_ISOLATE_PLUGINS !== 'false'
       ? loadNxPluginInIsolation
       : loadNxPlugin;