shell scripts: fix use of dash at variable assignments

shell scripts: fix use of dash at variable assignments

When update-vim.sh was interpreted by the dash shell, the following
error occured:

    export: 100: 5: bad variable name

This was caused by the assignemt of the date, which had spaces and
colons in its string, via the export command.

For consistency surround all variable assignments with parentheses
except of the ones like this, where the variable is set to the output of
a command without using export, because this seems to be safe:

    VARIABLE=$(command)