Fix compilation for wasm: env WASI_SYSROOT should be optional

[NobodyXu]

On ubuntu-latest github action runner, it used to work without the environment variable.

Fixed #1109 #1113

[NobodyXu]

cc @Niederb @Kuuuube Can you try this PR please?

cc @antaalt

[Kuuuube]

I'm still able to replicate the issue using this pr.

[Kuuuube]

Noticed that #1105 had this diff

-                } else if target == "wasm32-wasi"
-                    || target == "wasm32-unknown-wasi"
-                    || target == "wasm32-unknown-unknown"
-                {

The wasm32-unknown-wasi and wasm32-unknown-unknown targets were not added to the targets list. Adding them to the list along with your changes in this PR does fix the issue. (without your PR's changes it throws errors by adding those targets)

Threw in a PR targeting this PR's branch. #1115

If theres anything wrong with my changes in that PR feel free to close it or do whatever.