Golang-Migrate not able to recognize dollar symbol db.runCommand - json

Question

While running db.runCommand it is not able to recognize dollar symbol and therefore treating $answer as string and not actual value.
[
{
"update": "userfeedback",
"updates": [
{
"q": {
"userId": 8426,
"questionIdentifier": "resumeLink"
},
"u": {
"$set": {
"answer": [
{
"resumeLink": "$answer",
"resumeId": "$UUID().hex().match(/^(.{8})(.{4})(.{4})(.{4})(.{12})$/).slice(1,6).join('-')",
"uploadSizeInByte": -1,
"source":"manual",
"dateUploaded": "$updatedAt"
}
]
}
}
}
]
}
]
Output: dollar symbol is not recognized.
[
{
"resumeLink" : "$answer",
"resumeId" : "$UUID().hex().match(/^(.{8})(.{4})(.{4})(.{4})(.{12})$/).slice(1,6).join('-')",
"uploadSizeInByte" : -1,
"source" : "manual",
"dateUploaded" : "$updatedAt"
}
]
While using updateMany query the similar query works
Update Query:
db.getCollection('userfeedback').updateMany(
{userId:8426, questionIdentifier:"resumeLink"},
[{
"$set": {
answer: [{
"resumeLink": "$answer",
"resumeId": UUID().hex().match(/^(.{8})(.{4})(.{4})(.{4})(.{12})$/).slice(1,6).join('-'),
"uploadSizeInByte": -1,
"source":"manual",
"dateUploaded": "$updatedAt"
}]
}
}]
)
Result:
[
{
"resumeLink": "https://cdn.upgrad.com/resume/asasjyotiranjana11.docx",
"dateUploaded": "2051-04-26T14:30:00.000Z",
"uploadSizeInByte": 644234,
"resumeId": "7fa1478d-478f-4869-9c4b-7ca8c0b9434g",
"source": "hiration"
}
]
can some one help me how to get same result with runCommand. thanks in advance