Install project directly in "Test Main" workflow (#36)

Replace the "Install dependencies" step with "Upgrade pip" to avoid
installing pinned dependencies from `requirements-ci.txt` for all
test environments across different Python versions.

This prevents dependency installation errors like:
  ERROR: No matching distribution found for numpy==2.3.1

This way, the project will be installed in each container the same
way it would normally be installed locally, which ensures correct
testing of the main branch for release readiness.

Author: pavelacamposp

.github/workflows/test_main.yml

@@ -34,10 +34,8 @@ jobs:
             sleep 5; \
           done
 
-      - name: Install dependencies
-        run: |
-          python -m pip install --upgrade pip
-          pip install --default-timeout=60 --retries=5 -r requirements-ci.txt
+      - name: Upgrade pip
+        run: python -m pip install --upgrade pip
 
       - name: Install project package in editable mode
         run: |
@@ -73,10 +71,8 @@ jobs:
             sleep 5; \
           done
 
-      - name: Install dependencies
-        run: |
-          python -m pip install --upgrade pip
-          pip install --default-timeout=60 --retries=5 -r requirements-ci.txt
+      - name: Upgrade pip
+        run: python -m pip install --upgrade pip
 
       - name: Install project package in editable mode
         run: |