env_project_file correctly handle directories

On 1.7 and 1.8, if you provide a directory to `env_project_file`, it will return `true` even if there is a project file in the directory.

Note that this PR is against the release-1.8: it was fixed on master by #44318, so we could also backport that one.

base/loading.jl

@@ -423,14 +423,14 @@ function env_project_file(env::String)::Union{Bool,String}
         project_file === nothing || return project_file
     end
     if isdir(env)
+        project_file = true
         for proj in project_names
             maybe_project_file = joinpath(env, proj)
             if isfile_casesensitive(maybe_project_file)
                 project_file = maybe_project_file
                 break
             end
         end
-        project_file =true
     elseif basename(env) in project_names && isfile_casesensitive(env)
         project_file = env
     else