TT#101060 Build-Depend on either yarnpkg or yarn + use according binary

Debian/buster provides yarnpkg, but as of Debian/bullseye
it's named yarn (as provided by upstream).

Recent yarn versions also depend on node.js:

| Yarn requires Node.js 4.0 or higher to be installed.

Correspondingly Build-Depend on yarnpkg | nodejs (>= 12.19.0~) (to
support buster + bullseye at the same time), using the nodejs package
version as present in current Debian/testing (bullseye) as base.

Change-Id: I311e814beb3ca3f606b543e6116476f222276d78

debian/control

@@ -6,7 +6,8 @@ Homepage: https://www.sipwise.com/
 Standards-Version: 3.9.8
 Build-Depends:
  debhelper-compat (= 12),
- yarnpkg,
+ yarnpkg | nodejs (>= 12.19.0~),
+ yarnpkg | yarn,
 
 Package: ngcp-csc-ui
 Architecture: all

debian/rules

@@ -3,11 +3,14 @@
 # Uncomment this to turn on verbose mode.
 # export DH_VERBOSE=1
 
+# if yarnpkg is available use it, otherwise fall back to yarn
+YARN_BIN := $(shell which yarnpkg || echo yarn)
+
 %:
 	dh "$@"
 
 override_dh_auto_install:
-	yarnpkg install
+	$(YARN_BIN) install
 	cp src/config.template.js src/config.js
-	yarnpkg run build
+	$(YARN_BIN) run build
 	mv dist/spa csc