NuGet updater should specify a value for $TMP and $TEMP

[brettfo]

Found during an internal log audit.

An internal project with a custom target was trying to copy a file with the following:

...
<Copy SourceFiles="some_file.txt" DestinationFiles="$(TEMP)/some_file.txt" />
...

But the updater image doesn't have the environment variable $TEMP defined so an attempt was made to copy the file to the root of the image which isn't allowed.

The Dockerfile should specify a value for both $TMP and $TEMP (and possibly $TMPDIR) so operations like this can succeed.