fix: Prefix tmp directory for agent download with "coder"

[kylecarbs]

This makes it easier to find the temporary dir for the coder binary.

[codecov]

Codecov Report

Merging #1038 (e4822e1) into main (88e30be) will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
- Coverage   67.54%   67.29%   -0.26%     
==========================================
  Files         259      259              
  Lines       15059    15059              
  Branches      151      151              
==========================================
- Hits        10172    10134      -38     
- Misses       3873     3903      +30     
- Partials     1014     1022       +8     

Flag	Coverage Δ
unittest-go-macos-latest	53.40% <ø> (-0.41%)	⬇️
unittest-go-postgres-	66.65% <ø> (+<0.01%)	⬆️
unittest-go-ubuntu-latest	56.16% <ø> (-0.24%)	⬇️
unittest-go-windows-2022	53.01% <ø> (+<0.01%)	⬆️
unittest-js	68.20% <ø> (ø)

Impacted Files	Coverage Δ
provisionersdk/agent.go	100.00% <ø> (ø)
pty/start_other.go	64.70% <0.00%> (-23.53%)	⬇️
peerbroker/dial.go	77.04% <0.00%> (-6.56%)	⬇️
cli/cliui/agent.go	77.19% <0.00%> (-5.27%)	⬇️
coderd/parameter/compute.go	74.07% <0.00%> (-4.45%)	⬇️
peer/conn.go	77.41% <0.00%> (-3.81%)	⬇️
provisioner/echo/serve.go	54.40% <0.00%> (-2.40%)	⬇️
cli/cliui/provisionerjob.go	76.42% <0.00%> (-2.15%)	⬇️
peer/channel.go	83.23% <0.00%> (-1.74%)	⬇️
peerbroker/proxy.go	57.55% <0.00%> (-0.59%)	⬇️
... and 4 more

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