HCL2 validation fails with: Extra characters after interpolation expression

### Nomad version
Output from `nomad version`
Nomad v0.12.7 (6147cb578794cb2d0c35d68fe1791728a09bb081) ( my production env ) **Not** affected
Nomad v1.1.3 (8c0c8140997329136971e66e4c2337dfcf932692) affected
Nomad v1.1.4 affected
Nomad v1.1.5 (117a23d2cdf26a1837b21c84f30c8c0f3441e927) affected

### Operating system and Environment details
Ubuntu 18.04 and 20.04

### Issue
hello,

If a client meta key starts with a number, it breaks the hcl2 validator for a job, hcl1 appears to work fine.

### Reproduction steps
* nomad agent config for this example:
```
client {
enabled = true
meta {
"56net" = "true"
"web" = "true"
"balancer" = "false"
}
}
```
* a dummy job file
```
# vi: tabstop=4 shiftwidth=4 expandtab
job "ajob" {
datacenters = ["dc1"]
type = "service"

constraint {
attribute = "${meta.56net}" /* <== this one */
value = "true"
}

group "ahappylittlegroup" {
count = 1
constraint {
operator = "distinct_hosts"
value = "true"
}
task "dummy" {
driver = "docker"
config {
image = "whatever"
network_mode="host"
}
resources {
memory = 2048
cpu = 400
}
}
}
}
```
* Execution
`nomad job validate dummy.nomad`

#### Expected Result
`Job validation successful` ( with green happy characters )

#### Actual Result
```
Error getting job struct: Failed to parse using HCL 2. Use the HCL 1 parser with `nomad run -hcl1`, or address the following issues:
dummy.nomad:7,33-36: Extra characters after interpolation expression; Expected a closing brace to end the interpolation expression, but found extra characters.
```

I also posted this at `https://discuss.hashicorp.com/t/extra-characters-after-interpolation-expression/29726`
and another user was able to reproduce this as well.

Thank you

Start by reproducing the issue with the supplied client meta configuration and dummy job, then run `nomad job validate dummy.nomad` against the affected Nomad versions. Done means the HCL2 validator accepts `${meta.56net}` and reports successful job validation without requiring the HCL1 option.

Written by the indexing model from the issue text.