Enable multiline outputs

Simply copying set_env for now.

Fixes #31

src/GitHubActions.jl

@@ -130,7 +130,14 @@ set_command_echo(enable) = command("echo", (), enable ? "on" : "off")
 
 Set the output with name `k` to value `v`.
 """
-set_output(k, v) = add_to_file("GITHUB_OUTPUT", "$k=$v")
+function set_output(k, v)
+    val = cmd_value(v)
+    delimiter = "EOF"
+    while occursin(delimiter, val)
+        delimiter *= "EOF"
+    end
+    add_to_file("GITHUB_OUTPUT", join(["$k<<$delimiter", val, delimiter], "\n"))
+end
 
 """
     set_secret(v)

test/runtests.jl

@@ -38,9 +38,16 @@ const GHA = GitHubActions
     @test (@capture_out set_command_echo(false)) == "::echo::off\n"
 
     mktemp() do file, io
-        withenv("GITHUB_OUTPUT" => file) do
+        withenv("GITHUB_OUTPUT" => file, map(c -> string(c) => nothing, 'a':'z')...) do
             set_output("a", "b")
-            @test read(file, String) == "a=b\n"
+            @test read(file, String) == "a<<EOF\nb\nEOF\n"
+            set_output("b", "fooEOFbar")
+            @test read(file, String) == "a<<EOF\nb\nEOF\nb<<EOFEOF\nfooEOFbar\nEOFEOF\n"
+            rm(file)
+            set_output("c", [])
+            @test read(file, String) == "c<<EOF\n[]\nEOF\n"
+            set_output("d", nothing)
+            @test read(file, String) == "c<<EOF\n[]\nEOF\nd<<EOF\n\nEOF\n"
         end
     end