Fix --level reviewdog option when fail_on_error is false

[daniele-pini]

Description

According to the definition of the core.getInput function, the return type is a string:

/**
 * Gets the value of an input.
 * Unless trimWhitespace is set to false in InputOptions, the value is also trimmed.
 * Returns an empty string if the value is not defined.
 *
 * @param     name     name of the input to get
 * @param     options  optional. See InputOptions.
 * @returns   string
 */
export function getInput(name: string, options?: InputOptions): string {

The default assigned to the value is also a string:

  fail_on_error:
    description: |
      Exit code for reviewdog when errors are found [true,false]
      Default is `false`.
    required: false
    default: "false"

Except fail_on_error was used like a boolean in code. This PR fixes the comparison to check against the 'true' string value.