fix: Parsing dynamic values for agent results in error

[kylecarbs]

The logic required a constant value before, which disallowed dynamic
value injection into the agent. This isn't an accurate limitation,
so inverting the logic resolves it.

[codecov]

Codecov Report

Merging #564 (73e9793) into main (a06821c) will increase coverage by 0.10%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main     #564      +/-   ##
==========================================
+ Coverage   63.00%   63.10%   +0.10%     
==========================================
  Files         194      108      -86     
  Lines       11107    10024    -1083     
  Branches       85        0      -85     
==========================================
- Hits         6998     6326     -672     
+ Misses       3354     2979     -375     
+ Partials      755      719      -36     

Flag	Coverage Δ
unittest-go-	61.84% <50.00%> (+0.17%)	⬆️
unittest-go-macos-latest	57.56% <0.00%> (-0.14%)	⬇️
unittest-go-ubuntu-latest	60.79% <50.00%> (+0.03%)	⬆️
unittest-go-windows-2022	?
unittest-js	?

Impacted Files	Coverage Δ
provisioner/terraform/provision.go	75.27% <50.00%> (+0.67%)	⬆️
pty/ptytest/ptytest.go	94.82% <0.00%> (-5.18%)	⬇️
cli/projectinit.go	56.36% <0.00%> (-3.64%)	⬇️
coderd/workspaceresources.go	57.60% <0.00%> (-3.23%)	⬇️
provisioner/echo/serve.go	54.40% <0.00%> (-2.40%)	⬇️
provisionerd/provisionerd.go	80.32% <0.00%> (-0.66%)	⬇️
site/src/components/Form/FormCloseButton.tsx
site/src/components/Header/HeaderButton.tsx
site/src/components/SignIn/CliAuthToken.tsx
site/src/theme/global-fonts.ts
... and 87 more

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