fix(ci): honour FORCE_REBUILD from both manual trigger forms

The escape hatch added in #9 only read a bare $FORCE_REBUILD, which covers
`crow pipeline start -p FORCE_REBUILD=1`, where the parameter is injected
into the step environment. It does not cover
`crow pipeline create --branch main --var FORCE_REBUILD=1`, which passes a
crow substitution variable instead.

Triggering a fresh run of main is a `create`, so the flag would have been
silently ignored in exactly the case it is needed, and the run would skip
every Alpine version it was meant to rebuild.

Read the substituted ${FORCE_REBUILD} into FORCE_REBUILD_VAR and accept
either source. Crow blanks an unsupplied substitution variable rather than
failing, as recorded in 8eacc0a, so a plain cron run still takes the dedup
path.

.crow/build.yaml

@@ -181,10 +181,18 @@ steps:
           # Escape hatch for when the builder image itself changes in a way that
           # alters the resulting R binary (a newly enabled graphics device, a
           # different BLAS). The artifact filename carries only the R version, so
-          # dedup would otherwise keep the stale binaries forever. Set FORCE_REBUILD
-          # to a non-empty value as a pipeline variable on a manual run, or hardcode
-          # it here temporarily; uploads overwrite in place.
-          if [ -n "$FORCE_REBUILD" ]; then
+          # dedup would otherwise keep the stale binaries forever. Uploads
+          # overwrite in place.
+          #
+          # The two ways to trigger a manual run pass the flag differently:
+          #   crow pipeline create --branch main --var FORCE_REBUILD=1
+          #     -> a crow substitution variable, read below as ${FORCE_REBUILD},
+          #        which crow blanks out when it was not supplied
+          #   crow pipeline start -p FORCE_REBUILD=1
+          #     -> injected into the step environment, read as bare $FORCE_REBUILD
+          # Accept either, so the flag cannot be silently ignored.
+          FORCE_REBUILD_VAR="${FORCE_REBUILD}"
+          if [ -n "$FORCE_REBUILD_VAR" ] || [ -n "$FORCE_REBUILD" ]; then
             echo "FORCE_REBUILD is set, ignoring artifacts already in the bucket"
           else
             for VERSION in $(cat r-versions-to-build.txt); do