Add callout to PrivateLink docs about Environment Variables (#4701)

## What are you changing in this pull request and why?
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This PR adds a callout snippet regarding the limitation in terms of
using custom Environment Variables to populate the `hostname` of a
PrivateLink endpoint (for the warehouses that support it). This is not
supported in dbt Cloud and the suggested workaround is to use Extended
Attributes.

Instructions were updated for:
- Databricks
- Redshift
- Postgres

Instructions were _not_ updated for:
- Snowflake
- VCS

[MUL-512](https://dbtlabs.atlassian.net/browse/MUL-512)


![image](https://github.com/dbt-labs/docs.getdbt.com/assets/4997781/69f207e1-e288-405f-a118-69fa4624674c)



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[MUL-512]:
https://dbtlabs.atlassian.net/browse/MUL-512?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

website/docs/docs/cloud/secure/about-privatelink.md

@@ -6,10 +6,11 @@ sidebar_label: "About PrivateLink"
 ---
 
 import SetUpPages from '/snippets/_available-tiers-privatelink.md';
+import PrivateLinkHostnameWarning from '/snippets/_privatelink-hostname-restriction.md';
 
 <SetUpPages features={'/snippets/_available-tiers-privatelink.md'}/>
 
-PrivateLink enables a private connection from any dbt Cloud Multi-Tenant environment to your data platform hosted on AWS using [AWS PrivateLink](https://aws.amazon.com/privatelink/) technology. PrivateLink allows dbt Cloud customers to meet security and compliance controls as it allows connectivity between dbt Cloud and your data platform without traversing the public internet. This feature is supported in most regions across NA, Europe, and Asia, but [contact us](https://www.getdbt.com/contact/) if you have questions about availability. 
+PrivateLink enables a private connection from any dbt Cloud Multi-Tenant environment to your data platform hosted on AWS using [AWS PrivateLink](https://aws.amazon.com/privatelink/) technology. PrivateLink allows dbt Cloud customers to meet security and compliance controls as it allows connectivity between dbt Cloud and your data platform without traversing the public internet. This feature is supported in most regions across NA, Europe, and Asia, but [contact us](https://www.getdbt.com/contact/) if you have questions about availability.
 
 ### Cross-region PrivateLink
 
@@ -24,3 +25,5 @@ dbt Cloud supports the following data platforms for use with the PrivateLink fea
 - [Redshift](/docs/cloud/secure/redshift-privatelink)
 - [Postgres](/docs/cloud/secure/postgres-privatelink)
 - [VCS](/docs/cloud/secure/vcs-privatelink)
+
+<PrivateLinkHostnameWarning features={'/snippets/_privatelink-hostname-restriction.md'}/>

website/snippets/_privatelink-hostname-restriction.md

@@ -0,0 +1,5 @@
+:::caution Environment variables
+
+Using [Environment variables](/docs/build/environment-variables) when configuring PrivateLink endpoints isn't supported in dbt Cloud. Instead, use [Extended Attributes](/docs/deploy/deploy-environments#extended-attributes) to dynamically change these values in your dbt Cloud environment.
+
+:::