Allow setting multi-tenant strategy through ENV variable

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**What problem will this feature request solve?**

We looked into use-tenants and wanted to share what we found.

It does work via config.yml (`config.settings.use-tenants: false`), so that part is correct. But it can't be set via environment variable: the setting is defined with `:can-read-from-env? false` (src/metabase/permissions/settings.clj), which means MB_USE_TENANTS is silently ignored.

The practical consequence for us is that neither option locks the setting. A value from config.yml is only re-applied at startup, and settings are shown as read-only in the admin UI only when they come from an environment variable. So an admin can still change it at any time.

**Describe the solution you'd like.**
`MB_TENANT_STRATEGY` = `true` or `false`

**How does this feature request impact you?**
We've got 7+ environments, and have had to click-ops to change to multi-tenant, plus we have to do this when we roll out new environments.

Start in src/metabase/permissions/settings.clj, where the multi-tenant setting and its environment-variable behavior are defined. Check the existing setting name and the requested MB_TENANT_STRATEGY versus MB_USE_TENANTS naming, then verify that an environment value controls the setting and makes it read-only in the admin UI.

Written by the indexing model from the issue text.