Try adding the existence check before reinstalling

cibuildwheel/macos.py

@@ -232,8 +232,8 @@ def setup_python(
         sys.exit(1)
 
     # ensure pip is installed
+    if not (installation_bin_path / "pip").exists():
         call(["python", "-m", "ensurepip"], env=env, cwd="/tmp")
-    # ensure we have the version that matches our constraints
         # use "--force-reinstall" ensures that it's installed as 'pip'
         call(
             [
@@ -248,6 +248,14 @@ def setup_python(
             env=env,
             cwd="/tmp",
         )
+
+    # ensure we have the version that matches our constraints
+    call(
+        ["python", "-m", "pip", "install", "--upgrade", "pip", *dependency_constraint_flags],
+        env=env,
+        cwd="/tmp",
+    )
+
     assert (installation_bin_path / "pip").exists()
     call(["which", "pip"], env=env)
     call(["pip", "--version"], env=env)

cibuildwheel/windows.py

@@ -176,8 +176,8 @@ def setup_python(
     log.step("Installing build tools...")
 
     # ensure pip is installed
+    if not (installation_path / "Scripts" / "pip.exe").exists():
         call(["python", "-m", "ensurepip"], env=env, cwd="C:\\cibw")
-    # ensure we have the version that matches our constraints
         # use "--force-reinstall" ensures that it's installed as 'pip.exe'
         call(
             [
@@ -192,6 +192,12 @@ def setup_python(
             env=env,
         )
 
+    # ensure we have the version that matches our constraints
+    call(
+        ["python", "-m", "pip", "install", "--upgrade", "pip", *dependency_constraint_flags],
+        env=env,
+    )
+
     assert (installation_path / "Scripts" / "pip.exe").exists()
     where_pip = (
         subprocess.run(