fix TARGET_FILTER corner case for SDKs

If a TARGET_FILTER is set only imagebuilder/sdk/rootfs for that target
are created. In case of the SDK multiple valid targets are in the same
row printed via `dump-target-info.pl`. With the previous logic the
"first" target is used expecting all targets to be on the same commit.
However when using TARGET_FILTER the specific target should be used for
the SDK, not just one that supports the same architecture.

A logic is added that if TARGET_FILTER is set, it's used as the SDK
target.

Signed-off-by: Paul Spooren <mail@aparcar.org>

.github/workflows/containers.yml

@@ -140,7 +140,7 @@ jobs:
           while read -r line;
           do
             ARCH=$(echo "$line" | cut -d " " -f 1)
-            TARGET=$(echo "$line" | cut -d " " -f 2)
+            [ -z "$TARGET_FILTER" ] && TARGET=$(echo "$line" | cut -d " " -f 2) || TARGET="$TARGET_FILTER"
             TARGETS=$(echo "$line" | cut -d " " -f 2- | sed -e 's/ /\\n/g')
 
             [[ $FIRST -ne 1 ]] && JSON="$JSON"','