Revert "Remove requirements directory and simplify pip install"

This reverts commit e88e1bc.

Author: taeold

.kokoro/build.sh

@@ -27,10 +27,9 @@ EOF
 ### Install dependencies
 pip install \
   --only-binary :all: \
-  twine \
-  build \
-  wheel \
-  keyrings.google-artifactregistry-auth \
+  --require-hashes \
+  --no-deps \
+  -r .kokoro/requirements/build.txt \
   --verbose # shows it is pulling from Airlock
 
 ### Build and Publish

.kokoro/requirements/build.in

@@ -0,0 +1,6 @@
+--only-binary :all:
+twine>=6.2.0
+build>=1.3.0
+wheel>=0.45.1
+keyring>=25.7.0
+keyrings.google-artifactregistry-auth>=1.1.2