Change environment variable from NPR_URL to NPR_API_URL

Update environment variable name for registry URL configuration to be more
descriptive and consistent with API naming conventions.

- Update RegistryPublishConfig to use NPR_API_URL instead of NPR_URL
- Update README documentation with correct environment variable name
- Update test comments for clarity

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: pditommaso

README.md

@@ -56,7 +56,7 @@ The registry publishing configuration supports multiple ways to provide the URL
 The registry URL can be configured via (in order of priority):
 1. `nextflowPlugin.publishing.registry.url` in build.gradle
 2. Gradle property: `-Pnpr.url=https://your-registry.com/api`
-3. Environment variable: `NPR_URL=https://your-registry.com/api`
+3. Environment variable: `NPR_API_URL=https://your-registry.com/api`
 4. Default: `https://plugin-registry.seqera.io/api`
 
 #### API Key
@@ -77,7 +77,7 @@ npr.apiKey=your-secret-api-key
 
 Using environment variables:
 ```bash
-export NPR_URL=https://my-custom-registry.com/api
+export NPR_API_URL=https://my-custom-registry.com/api
 export NPR_API_KEY=your-secret-api-key
 ./gradlew publishPlugin
 ```

src/main/groovy/io/nextflow/gradle/registry/RegistryPublishConfig.groovy

@@ -37,7 +37,7 @@ class RegistryPublishConfig {
         }
 
         // Try environment variable
-        def envVar = System.getenv('NPR_URL')
+        def envVar = System.getenv('NPR_API_URL')
         if (envVar) {
             return envVar
         }

src/test/groovy/io/nextflow/gradle/NextflowPluginTest.groovy

@@ -127,7 +127,7 @@ class NextflowPluginTest extends Specification {
                     return gradleProp.toString()
                 }
 
-                // Mock environment variable for test
+                // Mock environment variable for test (NPR_API_URL)
                 return 'https://env-registry.com/api'
             }
         }