Force strict channel priority when building images

This is best practice as described here: bioconda/bioconda-recipes#13774

The following command:
conda install -c conda-forge -c bioconda 'changeo=0.4.4' 'biopython=1.72' 'python=3.7.1' 'r-data.table=1.11.4' 'bash=4.4.18' 'xlrd=1.2.0' 'r-reshape2=1.4.3' 'r-seqinr=3.4_5' 'r-scales=0.5.0' 'tar=1.34' 'file=5.39' 'r-ggplot2=3.0.0' 'unzip=6.0' -p /usr/local --copy --yes --strict-channel-priority

Finishes in 10-15 seconds with strict channel priority. Never finishes (at least not within 3 hours) without it.

lib/galaxy/tool_util/deps/mulled/invfile.lua

@@ -90,7 +90,7 @@ inv.task('build')
             .. 'conda install '
             .. channel_args .. ' '
             .. target_args
-            .. ' -p /usr/local --copy --yes '
+            .. ' --strict-channel-priority -p /usr/local --copy --yes '
             .. verbose
             .. postinstall)
     .wrap('build/dist')