bindings: fix handling of env secrets in remote builds

Previously, using --secret=id=foo,env=BAR in remote mode would fail because the client sent the env var name to the server, which tried to resolve it locally. This patch modifies the client to resolve the environment variable locally, write it to a temp file, and send it as a file-based secret.

Fixes #27494

Signed-off-by: ZuhairM7 <ZuhairM7>
Signed-off-by: ZuhairM7 <[email protected]>