diff --git a/src/environment.ts b/src/environment.ts
index 9912f0382..03d4870be 100644
--- a/src/environment.ts
+++ b/src/environment.ts
@@ -172,4 +172,6 @@ export interface Env {
   getOptional(name: string): string | undefined;
   /** Gets the entries of the underlying `ProcessEnv`. */
   entries(): Array<[string, string | undefined]>;
+  /** Sets an environment variable. */
+  set(name: string, value: string): void;
 }
diff --git a/src/testing-utils.ts b/src/testing-utils.ts
index 748ce40ea..c6ff0dbda 100644
--- a/src/testing-utils.ts
+++ b/src/testing-utils.ts
@@ -331,11 +331,19 @@ export type ActionVarOverrides = Partial<
  * excluding some that are expected to be set to paths. See `setupActionsVars`.
  *
  * @param overrides Overrides for the defaults.
+ * @param env The environment to set the variables for.
  */
-export function setupBaseActionsVars(overrides?: ActionVarOverrides) {
+export function setupBaseActionsVars(
+  overrides?: ActionVarOverrides,
+  env?: Env,
+) {
   const vars = { ...DEFAULT_ACTIONS_VARS, ...overrides };
   for (const [key, value] of Object.entries(vars)) {
-    process.env[key] = value;
+    if (env) {
+      env.set(key, value);
+    } else {
+      process.env[key] = value;
+    }
   }
 }
 
diff --git a/src/util.ts b/src/util.ts
index 1e79382f6..36f0ce04b 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -572,6 +572,9 @@ export function getEnv(env: NodeJS.ProcessEnv = process.env): Env {
     getRequired: (name) => getRequiredEnvVar(env, name),
     getOptional: (name) => getOptionalEnvVarFrom(env, name),
     entries: () => Object.entries(env),
+    set: (name, value) => {
+      env[name] = value;
+    },
   };
 }