Add interposition fixes for use with APK Foundry

See merge request !5

Occasionally, a package will use the $srcdir environment variable for
its own purposes and change its behavior during build if it is set.
This is not normally a problem since $srcdir is not normally exported by
abuild. However, if $srcdir is inherited from the environment in order
to override its default value of $startdir/src, it will continue to be
inherited and thus confuse those packages. Instead, check for
$ABUILD_SRCDIR. Likewise for $pkgbasedir and $ABUILD_PKGBASEDIR for
consistency sake.

This is the only appearance of `apk` in the source.  Appears accidental.

Fixes: 41343329 ("abuild: fix dependency tracing for cross builds")
Signed-off-by: A. Wilcox <AWilcox@Wilcox-Tech.com>

abuild, abuild-keygen: drop echo -n as well

See merge request !4

tests: fix newapkbuild tests

See merge request !2

* Do not use the -n option for newapkbuild_simple_test: since commit
  a68354eb, this option is not allowed
  when providing PKGNAME[-PKGVER] (i.e. not SRCURL) as the final
  argument.
* Use newapkbuild from the working tree, not whatever is in $PATH.

POSIX me harder

See merge request !3

According to POSIX, the "command -v" form of command(1) accepts only a
single argument (command_name). Some shells will check additional
arguments, but not dash.