buildrpm: fix embarassing bug in shell variable expansions
`$distance_$node` looks up and concatenates `$distance_` and
`$node`. `$distance_` is empty, so we were getting the node without
the distance. Using the curly braces makes our intent explicit and
produces better-versioned RPMS.
Differential Revision: https://phab.mercurial-scm.org/D4493
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