Switch to quilt for PGDG package modifications

[jasonmp85]

build-pgdebs makes several modifications to the base PGDG PostgreSQL packages in order to ensure the isolation tester, vanilla tests, regress.so, etc. are all in place at test-time. While it's OK to script changes to files within the debian subdirectory, edits of the source code itself really should be performed with quilt.

The following PostgreSQL source modifications should be performed using quilt patches:

• Fix test install directory for vpath builds
  

  the-process/circleci/images/debbuilder/files/build-pgdebs

  Lines 69 to 71 in be26e47

  	# shellcheck disable=2016
  	sed -i 's/file ||/{file#"$(srcdir)"} ||/' src/test/regress/GNUmakefile
  	tweak+=("Fix bug with test installation during vpath builds")

• Add the isolation tester to installed files
  

  the-process/circleci/images/debbuilder/files/build-pgdebs

  Lines 81 to 85 in be26e47

  	sed -i '/^install: | all$/ {
  	r /tmp/install-isolation.mk
  	d
  	}' src/test/isolation/Makefile
  	tweak+=("Add PostgreSQL's isolation tester to installed files")

The vpath issue is actually a bug upstream and should be reported.