fix: made build target to work predictably

[defanator]

Proposed changes

Previously make build did not do anything if a binary was deleted from the build directory, or the directory was created before running the target, as the target was not marked as PHONY:

% make clean
rm -rf ./build
% make build
GOWORK=off CGO_ENABLED=0 go build -ldflags="-w -X main.version=v2.23.1 -X main.commit=7a08fe56 -X main.date=2023-03-10_18-46-36" -o ./build/nginx-agent
% make build
make: `build' is up to date.
% rm build/nginx-agent 
% make build
make: `build' is up to date.
% ls -l build 
total 0

After the change:

% make clean
rm -rf ./build
% make build
GOWORK=off CGO_ENABLED=0 go build -ldflags="-w -X main.version=v2.23.1 -X main.commit=efc82b89 -X main.date=2023-03-10_19-53-49" -o ./build/nginx-agent
% make build
make: Nothing to be done for `build'.
% rm build/nginx-agent 
% make build
GOWORK=off CGO_ENABLED=0 go build -ldflags="-w -X main.version=v2.23.1 -X main.commit=efc82b89 -X main.date=2023-03-10_19-54-14" -o ./build/nginx-agent

(simply marking it as PHONY makes no sense as in usual workflows one would never need to rebuild the same binary in subsequent calls)

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• I have updated any relevant documentation (README.md)

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