Dynamically adding read-only attribute to lightning-input element in lwc framework

The usual way to address this is to hold the "read-only-ness" as tracked state (a change to it causes refresh of the UI) and to then conditionally render the input with or without the required read-only flag.

app.html

<template>
    <div class="slds-grid">
        <div class="slds-col">
            <lightning-radio-group name="opportunityCreation"
            label="Create an Opportunity"
            options={options}
            value={createOpp}
            onchange={createOppHandler}>
            </lightning-radio-group>
        </div>
        <div class="slds-col">
            <template if:false={readOnly}>
                <lightning-input label="Opportunity Name" data-id="oppName">
                </lightning-input>
            </template>
            <template if:true={readOnly}>
                <lightning-input label="Opportunity Name" data-id="oppName" read-only>
                </lightning-input>
            </template>
        </div>
    </div>
</template>

app.js

import { LightningElement, track, api } from 'lwc';

export default class App extends LightningElement {

    @track options = [{'label': 'Yes', value:'yes'},{'label':'No', value:'no'}];
    @track createOpp = 'yes';
    @track readOnly = false;

    createOppHandler(event) {
        this.createOpp = event.detail.value;
        this.readOnly = this.createOpp == "no";
    }
}

Also updated in your playground.

UPDATE:

I suspect the markup:

            <template if:false={readOnly}>
                <lightning-input label="Opportunity Name" data-id="oppName">
                </lightning-input>
            </template>
            <template if:true={readOnly}>
                <lightning-input label="Opportunity Name" data-id="oppName" read-only>
                </lightning-input>
            </template>

Can be replaced by:

            <lightning-input label="Opportunity Name" data-id="oppName" read-only={readOnly}>
            </lightning-input>

Though I haven't tested it. The technique I showed is, however, good for other cases where you want to pass different parameters rather than just different parameter values.