Specify -buildmode=c-shared after GO_FLAGS rather than before
Specify -buildmode=c-shared after GO_FLAGS rather than before

When building pam_fscrypt.so, specify -buildmode=c-shared after
$(GO_FLAGS) so that it overrides any user-specified buildmode.

This is needed to allow -buildmode=pie to be specified in GO_FLAGS if
the packager wants to build fscrypt as a position-independent executable
(e.g. following https://wiki.archlinux.org/title/Go_package_guidelines).
Previously, trying to do this caused pam_fscrypt.so to be incorrectly
built as an executable rather than as a shared library.

google/fscrypt,google/fscrypt,google/fscrypt