Avoid compilation in autopkgtest

Invoking compiler during autopkgtest triggers blhc(1) error about
missing hardening CFLAGS.

Since we are testing build system, not compiler, this patch replaces
compiler with cp(1).
8 jobs for master in 11 minutes and 32 seconds (queued for 21 minutes and 38 seconds)
latest
Status Job ID Name Coverage
  Build
passed #279922
build

00:04:50

 
  Test
passed #279927
autopkgtest

00:03:28

passed #279928
blhc

00:01:33

passed #279926
lintian

00:01:46

passed #279929
piuparts

00:03:41

passed #279925
reprotest

00:05:08

passed #279924
test-build-all

00:02:18

passed #279923
test-build-any

00:03:25