Proposal: honor `TMPDIR` in gdrcopy package build scripts

[hanwen-cluster]

Problem

packages/build-rpm-packages.sh and packages/build-deb-packages.sh create their build workspace with an absolute /tmp template:

tmpdir=`mktemp -d /tmp/gdr.XXXXXX`

Because the template is an absolute path, mktemp uses it verbatim and never consults TMPDIR. The entire build then runs under /tmp, where the RPM %build stage executes ./config_arch. On hosts that mount /tmp with noexec (common on hardened OS), that script can't execute, and the build fails:

make: ./config_arch: Permission denied
GDRAPI_ARCH=
...
undefined reference to `memcpy_uncached_store_neon'   # arch never detected → wrong sources compiled

There is no way to redirect the build off /tmp today.

Fix

Let mktemp resolve the base directory itself, so it honors TMPDIR and falls back to /tmp when unset:

-tmpdir=`mktemp -d /tmp/gdr.XXXXXX`
+tmpdir=`mktemp -d --tmpdir gdr.XXXXXX`

Apply the same change in both packages/build-rpm-packages.sh and packages/build-deb-packages.sh.

Portability note

--tmpdir is a GNU coreutils extension. These are Linux-only rpm/deb packaging scripts, so GNU mktemp is a safe assumption. If you prefer to avoid the GNU-specific flag, the portable equivalent is:

-tmpdir=`mktemp -d /tmp/gdr.XXXXXX`
+tmpdir=`mktemp -d "${TMPDIR:-/tmp}/gdr.XXXXXX"`

Both fixes are functionally identical.