Fill variable text with JS directly to speed up

It's too slow to use `set(value)`, often timing out.
Filling with JS is much faster for longer text, especially
when the key size is larger than 8192.

Before this patch:

```
Top 3 slowest examples (256.83 seconds, 89.4% of total time):
  cloning code using a deploy key user sets up a deploy key with QA::Runtime::Key::RSA(8192) to clone code using pipelines
    161.26 seconds ./qa/specs/features/project/deploy_key_clone_spec.rb:42
  cloning code using a deploy key user sets up a deploy key with QA::Runtime::Key::ECDSA(521) to clone code using pipelines
    47.79 seconds ./qa/specs/features/project/deploy_key_clone_spec.rb:42
  cloning code using a deploy key user sets up a deploy key with QA::Runtime::Key::ED25519() to clone code using pipelines
    47.79 seconds ./qa/specs/features/project/deploy_key_clone_spec.rb:42
```

Note that 161.26 was timed out. So it would actually take longer if
it could ever complete. After patch:

```
Top 3 slowest examples (166.72 seconds, 83.8% of total time):
  cloning code using a deploy key user sets up a deploy key with QA::Runtime::Key::RSA(8192) to clone code using pipelines
    83.66 seconds ./qa/specs/features/project/deploy_key_clone_spec.rb:42
  cloning code using a deploy key user sets up a deploy key with QA::Runtime::Key::ECDSA(521) to clone code using pipelines
    42.78 seconds ./qa/specs/features/project/deploy_key_clone_spec.rb:42
  cloning code using a deploy key user sets up a deploy key with QA::Runtime::Key::ED25519() to clone code using pipelines
    40.27 seconds ./qa/specs/features/project/deploy_key_clone_spec.rb:42
```

Not that faster for smaller keys, but it's much faster for RSA 8192
(2 times faster). This was inspired from:

https://github.com/teamcapybara/capybara/blob/679548cea10773d45e32808f4d964377cfe5e892/lib/capybara/selenium/node.rb#L217

Where it's clearing the field by filling an empty string. Here we
do the same for the exact value we want to fill.

qa/qa/page/project/settings/secret_variables.rb

@@ -23,7 +23,13 @@ module QA
             # After we fill the key, JS would generate another field so
             # we need to use the same index to find the corresponding one.
             keys[index].set(key)
-            all_elements(:ci_variable_input_value)[index].set(value)
+            node = all_elements(:ci_variable_input_value)[index]
+
+            # Simply run `node.set(value)` is too slow for long text here,
+            # so we need to run JavaScript directly to set the value.
+            # The code was inspired from:
+            # https://github.com/teamcapybara/capybara/blob/679548cea10773d45e32808f4d964377cfe5e892/lib/capybara/selenium/node.rb#L217
+            execute_script("arguments[0].value = #{value.to_json}", node)
           end
 
           def save_variables