[std.math] std.math.algebraic.cbrt is not pure

[thewilsonator]

Migrated from dlang/dmd#17272

void main() pure {
    import std.math: cbrt;
    auto x = cbrt(1.0);
}

gives

Error: `pure` function `D main` cannot call impure function `std.math.algebraic.cbrt`

[0xEAB]

Likely related to #10008