Resume interrupted downloads in shell cookbook

Problem:

In the shell cookbook, if a wget transfer fails or is interupted it will not be resumed in subsequent runs. If there is no checksum, execution will continue and the user will be presented with a confusing "This does not look like a tar archive" error:

First time:

cook - jansson fetch
--2023-11-25 03:33:51--  https://www.digip.org/jansson/releases/jansson-2.10.tar.gz
Resolving www.digip.org (www.digip.org)... 91.232.155.81, 2001:67c:1be8:1337::443
Connecting to www.digip.org (www.digip.org)|91.232.155.81|:443... connected.
ERROR: The certificate of 'www.digip.org' is not trusted.
ERROR: The certificate of 'www.digip.org' has expired.
The certificate has expired
make: *** [mk/ci.mk:24: ci-pkg] Error 5

Retry:

cook - jansson fetch
tar: This does not look like a tar archive
tar: Exiting with failure status due to previous errors
make: *** [mk/ci.mk:24: ci-pkg] Error 2

To recover, the user has to delete the corrupted source.tar.

I tried to fix this earlier by adding --continue in 749fd620. It was a mistake and this alone doesn't change anything for the shell version of the cookbook.

Changes introduced by this pull request:

We first download the file to source.tar.tmp, trying to resume the download if the file exist. After wget exists successfully we move the temporary file to source.tar.

This is the equivalent of this bit of cook.rs, but for the shell version of the cookbook.

cook.sh

@@ -88,7 +88,8 @@ function op {
                 then
                     if [ ! -f source.tar ]
                     then
-                        wget "$TAR" --continue -O source.tar
+                        wget "$TAR" --continue -O source.tar.tmp
+                        mv source.tar.tmp source.tar
                     fi
 
                     if [ -n "$TAR_SHA256" ]