Contract build: fix compatibility with mxpy v8

[andreibancioiu]

Fix invocation of mxpy contract build:

• https://github.com/multiversx/mx-ide-vscode/blob/master/src/sdk.ts#L246

# before v8
mxpy contract build foobar

vs.

# since v8
mxpy contract build --path foobar

For simplicity, we can break compatibility with mxpy v7 (this should be acceptable in practice).