pkgr: slow but fix builds on packager.io (#5840)

* Test

* Test

* My last try

* Avoid cleaning old Go version

.pkgr.yml

@@ -23,7 +23,7 @@ targets:
   centos-7:
     build_dependencies:
       - pam-devel
-      # required for go buildpack
+      # required for Go buildpack
       - perl-Digest-SHA
     dependencies:
       - pam

go.mod

@@ -68,4 +68,4 @@ require (
 	xorm.io/xorm v0.8.0
 )
 
-// +heroku goVersion go1.13.3
+// +heroku goVersion go1.13beta1