Testbed: fix apt pinning for old apt releases

When using apt < 2.0 in buster and earlier, autopkgtest expands source pinning (--pin-packages=somerelease=src:somepackage) into binary packages, because apt does not yet understand the "src:" notation. While doing so, it parses apt-cache dumpavail to map binary packages to their source packages. Rarely, apt includes a version here. For example, the source package for libgcc1 on stretch becomes "gcc-6 (6.3.0-18)". As this does not match "gcc-6", pinning it goes bad. Drop the version.