ci: speed up backend build and parallelize image publishing [CI]

backend/src/MoShiro.Backend/StaticSqlite.targets

@@ -15,12 +15,16 @@
            Text="Static SQLite is configured for the eight supported x64/arm64 Linux, musl, macOS and Windows targets. Unsupported RID: $(RuntimeIdentifier)." />
     <PropertyGroup>
       <_SqliteCache>$([System.IO.Path]::GetFullPath('$(BaseIntermediateOutputPath)sqlite/$(MoShiroSqliteSourceVersion)/'))</_SqliteCache>
+      <_SqliteCache Condition="'$(SqliteStaticCacheRoot)' != ''">$([System.IO.Path]::GetFullPath('$(SqliteStaticCacheRoot)/$(RuntimeIdentifier)/$(MoShiroSqliteSourceVersion)/$(SqliteStaticCacheKey)/'))</_SqliteCache>
       <_SqliteArchive>$(_SqliteCache)sqlite-amalgamation-$(MoShiroSqliteSourceVersion).zip</_SqliteArchive>
       <_SqliteSource>$(_SqliteCache)sqlite-amalgamation-$(MoShiroSqliteSourceVersion)/sqlite3.c</_SqliteSource>
       <_SqliteObjectDirectory>$(_SqliteCache)$(RuntimeIdentifier)/$(UseAotAnywhere)/</_SqliteObjectDirectory>
+      <_SqliteObjectDirectory Condition="'$(SqliteStaticCacheRoot)' != ''">$(_SqliteCache)$(UseAotAnywhere)/</_SqliteObjectDirectory>
       <_SqliteObject>$(_SqliteObjectDirectory)e_sqlite3.o</_SqliteObject>
       <_SqliteObject Condition="$(RuntimeIdentifier.StartsWith('win'))">$(_SqliteObjectDirectory)e_sqlite3.obj</_SqliteObject>
     </PropertyGroup>
+    <Error Condition="'$(SqliteStaticCacheRoot)' != '' and '$(SqliteStaticCacheKey)' == ''"
+           Text="SqliteStaticCacheKey is required when using SqliteStaticCacheRoot." />
     <MakeDir Directories="$(_SqliteObjectDirectory)" />
     <DownloadFile SourceUrl="https://www.sqlite.org/$(MoShiroSqliteSourceYear)/sqlite-amalgamation-$(MoShiroSqliteSourceVersion).zip"
                   DestinationFolder="$(_SqliteCache)" Condition="!Exists('$(_SqliteArchive)')" />