Fix codecov installation

[ajschmidt8]

#61 added an explicit TARGETPLATFORM build argument, which broke our CI images.

TARGETPLATFORM is a build argument provided by docker build automatically: https://docs.docker.com/engine/reference/builder/#automatic-platform-args-in-the-global-scope.

[ajschmidt8]

The TARGETPLATFORM argument is used to install codecov below:

• ci-imgs/Dockerfile

  Lines 87 to 106 in c1eedba

  	# Install codecov binary
  	RUN \
  	case "${TARGETPLATFORM}" in \
  	"linux/amd64") \
  	CODECOV_VERSION=v0.3.2 \
  	&& curl https://uploader.codecov.io/verification.gpg --max-time 10 --retry 5 \
  	| gpg --no-default-keyring --keyring trustedkeys.gpg --import \
  	&& curl -Os --max-time 10 --retry 5 https://uploader.codecov.io/${CODECOV_VERSION}/linux/codecov \
  	&& curl -Os --max-time 10 --retry 5 https://uploader.codecov.io/${CODECOV_VERSION}/linux/codecov.SHA256SUM \
  	&& curl -Os --max-time 10 --retry 5 https://uploader.codecov.io/${CODECOV_VERSION}/linux/codecov.SHA256SUM.sig \
  	&& gpgv codecov.SHA256SUM.sig codecov.SHA256SUM \
  	&& shasum -a 256 -c codecov.SHA256SUM \
  	&& chmod +x codecov \
  	&& mv codecov /usr/local/bin \
  	&& rm -f codecov* \
  	;; \
  	*) \
  	echo 'Codecov is only supported on "linux/amd64" machines'; \
  	;; \
  	esac