models: output: Make EnvironmentVariable a frozen model

EnvironmentVariable instances are never mutated after construction.
resolve_value() is a pure function returning a new string. Making
the model frozen enforces this invariant and enables hashing, which
allows using EnvironmentVariable in sets and as dict keys.

Reference: hermetoproject#1629 (comment)

Signed-off-by: Erik Skultety <eskultet@redhat.com>

Author: eskultety

hermeto/core/models/output.py

@@ -14,7 +14,7 @@
 log = logging.getLogger(__name__)
 
 
-class EnvironmentVariable(pydantic.BaseModel):
+class EnvironmentVariable(pydantic.BaseModel, frozen=True):
     """An environment variable high-level representation.
 
     An environment variable is represented as a string template that is evaluated (i.e.