Crowdin Action fails to detect existing pull request when using a non-main base branch and same-repo source branch

**Describe the bug**
When the base branch for the pull request is not main (e.g., develop), the Crowdin GitHub Action fails to detect an existing pull request from a same-repo source branch (e.g., l10n_crowdin_translations). As a result, it attempts to create a new pull request and fails with a 422 Validation Failed error.

The issue likely occurs because the action uses head=ORG_NAME:BRANCH when calling the GitHub API to check for existing PRs.

**To Reproduce**
Steps to reproduce the behavior:
1. Use the Crowdin GitHub Action with the following relevant configuration:
```yaml
localization_branch_name: l10n_crowdin_translations
pull_request_base_branch_name: develop
create_pull_request: true
```
2. Ensure that:
* A pull request already exists from l10n_crowdin_translations to develop
* Both branches exist in the same repository (not from a fork)

3. Trigger the workflow again (either via schedule or manual dispatch)

Example workflow:
```
name: Crowdin Action Pull

on:
workflow_dispatch:
schedule:
- cron: '0 7,12 * * *'

jobs:
synchronize-with-crowdin:
runs-on: ubuntu-latest
permissions:
contents: write
pull-requests: write

steps:
- name: Checkout
uses: actions/checkout@v4

- name: crowdin action
uses: crowdin/github-action@v2
with:
upload_sources: false
upload_translations: false
download_sources: true
download_translations: true
export_only_approved: true
localization_branch_name: l10n_crowdin_translations
pull_request_base_branch_name: develop
create_pull_request: true
push_sources: true
push_translations: true
pull_request_title: 'New Crowdin Translations'
pull_request_body: 'New Crowdin translations by [Crowdin GH Action](https://github.com/crowdin/github-action)'
pull_request_labels: 'Crowdin'
env:
GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
CROWDIN_PROJECT_ID: ${{ secrets.CROWDIN_PROJECT_ID }}
CROWDIN_PERSONAL_TOKEN: ${{ secrets.CROWDIN_PERSONAL_TOKEN }}
```

**Expected behavior**
The action should detect that a pull request from l10n_crowdin_translations to develop already exists and skip creating a new one. It should log PULL REQUEST ALREADY EXIST instead of failing.

**Screenshots**
Relevant log from the failing repo:
```
PUSH TO BRANCH l10n_crowdin_translations
[l10n_crowdin_translations 5b7daab] New Crowdin translations by GitHub Action
1 file changed, 21 insertions(+), 21 deletions(-)
To https://github.com/XXX/YYY.git
+ 27f2ff2...5b7daab l10n_crowdin_translations -> l10n_crowdin_translations (forced update)
CHECK IF PULL REQUEST ALREADY EXIST
CREATE PULL REQUEST
FAILED TO CREATE PULL REQUEST
RESPONSE: {
"message": "Validation Failed",
"errors": [
{
"resource": "PullRequest",
"code": "custom",
"message": "A pull request already exists for XXX:l10n_crowdin_translations."
}
],
"documentation_url": "https://docs.github.com/rest/pulls/pulls#create-a-pull-request",
"status": "422"
}
```

**Additional context**
The bug appears in this part of the Crowdin action logic:
```
pull_requests_response=$(curl ... "${PULLS_URL}?base=${BASE_BRANCH}&head=${ORG_NAME}:${BRANCH}")
```
When changing into it works
```
"${PULLS_URL}?base=${BASE_BRANCH}&head=${BRANCH}"
```
I've verified this by forking the action and applying the fix — and the problem was resolved. The action correctly detects existing pull requests and avoids trying to create duplicates.