This list template override formats all unordered list items using dashes when the unordered list is a child of any other list type, including task steps, so you might need to refine the XPath expression in the xsl:when test attribute.
<xsl:template match="*[contains(@class, ' topic/ul ')]/*[contains(@class, ' topic/li ')]">
<fo:list-item xsl:use-attribute-sets="ul.li">
<fo:list-item-label xsl:use-attribute-sets="ul.li__label">
<fo:block xsl:use-attribute-sets="ul.li__label__content">
<fo:inline>
<xsl:call-template name="commonattributes"/>
</fo:inline>
<xsl:choose>
<xsl:when test="ancestor::*[contains(@class, ' topic/li ')]">
<xsl:call-template name="insertVariable">
<xsl:with-param name="theVariableID" select="'Unordered List dash'"/>
</xsl:call-template>
</xsl:when>
<xsl:otherwise>
<xsl:call-template name="insertVariable">
<xsl:with-param name="theVariableID" select="'Unordered List bullet'"/>
</xsl:call-template>
</xsl:otherwise>
</xsl:choose>
</fo:block>
</fo:list-item-label>
<fo:list-item-body xsl:use-attribute-sets="ul.li__body">
<fo:block xsl:use-attribute-sets="ul.li__content">
<xsl:apply-templates/>
</fo:block>
</fo:list-item-body>
</fo:list-item>
</xsl:template>
And here's the variable from the strings file.
<variable id="Unordered List dash">-</variable>